Preheat your oven to 375°F and line a small loaf pan and a baking sheet with parchment paper.
In a skillet over medium heat, sauté the finely diced mushrooms, onion, and minced garlic until they are soft and the moisture has evaporated.
In a large mixing bowl, lightly mash the cooked lentils with a fork, leaving some texture for a better bite.
Add the sautéed vegetables, liquid egg whites, nutritional yeast, dried thyme, sea salt, and black pepper to the lentils, stirring until well combined.
Transfer the lentil mixture into the prepared loaf pan, pressing down firmly with a spatula to ensure it holds its shape.
On the baking sheet, toss the sliced zucchini and bell pepper with olive oil and a pinch of salt.
Place both the loaf and the vegetables in the oven; bake the loaf for 30 minutes until firm and the vegetables until tender and slightly charred.
During the last 5 minutes of baking, place the sourdough slice on the rack to toast until crisp.
Allow the loaf to rest for 5 minutes before slicing and serving alongside the roasted vegetables and toast.