Cook the lentils first. Add the lentils and vegetable broth to a pot and set it over medium heat. Once it comes to a simmer, cover the pot with a lid, leaving it tilted slightly so some steam can escape. Allow the lentils to simmer for 20-23 minutes until just tender but they still retain some bite. Drain off any excess broth then set aside.

Preheat the oven to 400 degrees. Line a large baking sheet with parchment. Remove and discard any stems that are still attached to the mushrooms and wipe off any dirt with a damp paper towel. Place the mushrooms on the baking sheet with the gills/stem side facing up. Drizzle each lightly with olive oil and season with salt. Roast for 10 minutes in the preheated oven. The mushrooms will release some liquid into the caps as they roast — leave it there as it will add moisture to the stuffing later on.

Once the lentils are done cooking and the mushrooms are in the oven, start the filling. Set a large pan or skillet or medium heat. Trim the dark green part and root end from the leek and discard. Cut the leek in half lengthwise then thinly slice. Rinse well to remove dirt and grit then pat dry. Scrub the carrot well or peel it, then finely chop. Add 1 tablespoon of olive oil to the pan. Once it shimmers, add the sliced leek and a pinch of salt. Cook, stirring every minute or so, until the leek is softened and just beginning to turn golden (this will take about 8 minutes). Add the carrot and cook for 5 minutes, stirring regularly. If the pan becomes dry at any point, add a splash or two of stock.

After the carrot has been cooking for 5 minutes, add the garlic and herbes de Provence and cook for 2 minutes, stirring regularly. Stir in the tomato paste and tamari. Cook for 2 minutes then add the cooked lentils, nutritional yeast, and finely chopped walnuts. Stir to combine, then cook for 2 more minutes before removing the pan from the heat. Fold in the breadcrumbs then taste the filling and add salt if needed.

Divide the filling between the mushroom caps then drizzle the tops with a little olive oil. Bake in the preheated oven for 10-15 minutes until warmed through. Garnish with finely chopped parsley then serve.