This is the comfortable middle ground between a stew and a pie. You get mushrooms, lentils, and potato without waiting for an assembled dish to finish in the oven.
Boil about 500 g peeled, evenly cut potatoes in water until tender. Drain and mash with a little olive oil and enough warmed unsweetened plant milk to soften the texture. Add the liquid gradually; different potatoes absorb it differently.
While they cook, brown 200 g sliced mushrooms in oil. Add a chopped onion and soften, then stir in a teaspoon tomato paste, 300 g cooked lentils, and about 200 ml vegetable stock. Simmer until the mixture is moist but no longer soupy. Season with pepper and a little soy sauce if wanted.
Serve in two deep bowls with the lentils spooned over or beside the mash. You should be able to draw a fork through both. If either part is stiff, loosen it before serving rather than expecting the other to compensate.