Prepare the Ingredients: Cook the quinoa and lentils according to package instructions. Finely chop the mushrooms and onion, and mince the garlic.
Sauté Vegetables: In a non-stick skillet, sauté the chopped onions, garlic, and mushrooms over medium heat until they release moisture and the onions are translucent. If necessary, add a small amount of vegetable broth to prevent sticking.
Mix Ingredients: In a food processor, pulse the rolled oats and walnuts (if using) until they have a coarse texture. Add the cooked quinoa, lentils, sautéed vegetables, ground flaxseed, nutritional yeast, tamari, tomato paste, smoked paprika, Italian seasoning, onion powder, black pepper, salt, and fennel seeds. Pulse until the ingredients are well combined but still retain some texture (don’t overprocess the mixture, or you will get a paste and mushy consistency).
Form the Sausages and chill: Form the sausages with your hands (using 1/3 cup of the mixture). If the mixture is too sticky, wet your hands or place it in the freezer for 7-10 minutes, and try again. Then, place the sausages on a serving plate lined with parchment paper and refrigerate them for 20 minutes to make them firm and not sticky.
Steam the Sausages: Bring water to a boil in a large pot and add a steamer basket. Wrap each sausage in parchment paper and tin foil, twisting the ends to seal*. Place the sausages in the steamer basket and steam for 30-35 minutes.
Cook the Sausages: Heat a cast iron pan over medium heat. Add the steamed sausages and cook on each side for 5-7 minutes until they are firm and slightly browned.
Cool and Serve: Let the sausages cool for a few minutes before serving. They can be enjoyed on their own, in a bun, or as a part of a larger meal.