Anybody who has eaten vegetarian food in mixed company has fielded the protein question. It is asked with concern, usually by somebody who has never calculated their own intake, and it is generally the wrong question.
The right one is more interesting and it does change how you cook.
None of the below is medical advice. Anyone with a specific health situation should speak to somebody qualified rather than reading a food article.
Why the usual question is not the useful one
Traditional vegetarian cuisines have fed enormous populations for a very long time, which is reasonable evidence that the basic problem is solved.
The solution in most of them is the same: pulses eaten with grains, in quantity, daily. Dal with rice, beans with maize, chickpeas with wheat. This combination appears independently across the world and it is not an accident.
Where the modern problem arises is not in traditional vegetarian eating. It is in vegetarian eating that has drifted away from the pulse-and-grain core toward vegetables, dairy and refined carbohydrate, which is a genuinely different pattern with a different profile.
So the question is not whether vegetarian food can supply protein. It is whether a particular person's vegetarian eating actually includes the parts that do.
The complete protein argument, which was overstated
There was a widely repeated idea that plant proteins are incomplete and must be carefully combined at each meal to be useful.
The strict version of that has been substantially walked back. The body maintains a pool of amino acids, and combining across a day rather than within a single meal appears to be sufficient for most people.
Which removes the anxious arithmetic that put a lot of people off. Eating pulses and grains regularly across the day does the job without anybody needing to engineer individual plates.
The underlying observation that different plant sources have different amino acid profiles remains true. The conclusion drawn from it was too strong.
The cooking question that follows
Here is where it becomes practical rather than nutritional.
If pulses are the centre of the eating, then a household needs a lot of ways to cook them that are not boring. That is a culinary problem, not a nutritional one, and it is where most vegetarian eating actually fails.
The households I know that eat this way well have maybe fifteen distinct pulse preparations in rotation. Different pulses, different treatments — soupy, dry, ground into batter, sprouted, fried, made into fritters, cooked into a stew with vegetables.
The ones that struggle have two or three, cook them the same way, and get bored, at which point the pulses quietly drop out and the eating drifts toward cheese and bread.
What actually varies between preparations
Worth listing because it is more than the pulse itself.
Consistency, from thin and soupy to entirely dry. The same lentil produces completely different dishes at different water levels.
Whether it is whole, split, or ground. Whole pulses hold their shape and eat like a stew. Split ones break down. Ground ones become batters and doughs and fritters.
Whether it is sprouted, which changes texture and flavour substantially and takes two days of doing nothing.
The souring agent, which is what most distinguishes regional versions of what is otherwise the same dish.
And the tempering, which can completely change the character of an identical base.
Dairy as a crutch
A pattern I noticed in my own eating and have since seen in others.
When cooking gets tiring, vegetarian meals drift toward dairy, because cheese and yoghurt are effortless and satisfying.
There is nothing wrong with dairy and there is something wrong with it replacing the pulse component, because the resulting pattern is different in ways that have nothing to do with anybody's opinions about it.
The practical guard is simply having cooked pulses available. This is the argument for the batch cooking approach — a pot of something in the fridge on Sunday means the drift does not happen on Wednesday.
What I would say if asked again
That the traditional answer is pulses and grains, eaten daily, in quantity.
That the risk is not the diet, it is the cook getting bored of the two dishes they know.
And that anybody genuinely concerned should look at what a week of their own eating contains rather than at what a category theoretically provides, which is advice that applies at least as much to the people asking the question.
Sprouting, which is easier than it sounds
One technique worth mentioning because it costs nothing and adds a whole category of preparations.
Soak a pulse overnight, drain it, and leave it damp in a covered container for a day or two, rinsing once or twice. It sprouts.
The texture changes completely — crisper, lighter — and it can be eaten raw in salads, steamed briefly, or cooked as normal.
It also shortens cooking time considerably, which is a practical benefit independent of anything else.
The main thing to watch is temperature and rinsing, since a warm container left unrinsed for two days can go wrong. Rinse morning and evening and it is straightforward.