Bring 1 cup of water to a boil in a small saucepan.
Add the lentils and 1/2 teaspoon salt. Simmer for about 20 minutes, until the lentils are tender but still hold their shape. They will finish cooking in the sauce.
Meanwhile, finely dice the onion, chop the garlic, thinly slice the celery, and slice the mushrooms.
Heat the olive oil in a large skillet over medium heat.
Add the onion, garlic, celery, and oregano. Cook for several minutes until the onion becomes soft and translucent.
Add the mushrooms and cook until they release their moisture and it evaporates.
Pour in the tomato passata.
Add the cooked lentils, dried basil, and balsamic vinegar.
Simmer for about 15 minutes, stirring occasionally.
Season with salt and black pepper to taste.
Meanwhile, cook the spaghetti according to the package instructions.
Drain and divide between serving bowls.
Top with the lentil Bolognese sauce and finish with plenty of vegetarian Parmesan.