Preheat your oven to 375°F and line a small loaf pan with parchment paper for easy removal.
Combine the ground flaxseed and water in a small bowl, stirring well and letting it sit for 5 minutes to create a thick gel.
Finely chop the cremini mushrooms and sauté them in a non-stick skillet over medium heat until they release their moisture and turn golden brown.
Place the cooked lentils, seitan, sautéed mushrooms, and walnuts into a food processor and pulse several times until the mixture is chopped and combined but still retains some texture.
Transfer the mixture to a large mixing bowl and add the nutritional yeast, oat flour, tomato paste, tamari, garlic powder, onion powder, sea salt, black pepper, and the flax gel.
Mix everything thoroughly with a spatula until a thick, cohesive dough forms.
Press the mixture firmly into the prepared loaf pan, using the back of a spoon to smooth the top and ensure there are no air pockets.
Bake for 35 to 40 minutes until the top is deeply browned and the loaf feels firm when pressed.
Remove from the oven and allow the loaf to cool in the pan for at least 10 minutes to set before slicing into thick portions.