Place lentils and water in a small saucepan.

Bring to a boil, then lower the heat and simmer, partially covered, for about 30 minutes or until the lentils are soft and the liquid is absorbed.

Transfer the cooked lentils to a medium bowl.

Add cider vinegar to the lentils and mash well.

Heat peanut or olive oil in a skillet over medium heat.

Add finely minced onion and garlic to the skillet and sauté until softened and fragrant, about 5 minutes.

Add minced mushrooms to the skillet and cook until they release their moisture and start to brown, about 7-10 minutes.

Transfer the cooked onion, garlic, and mushroom mixture to the bowl with the mashed lentils.

Add very finely minced walnuts, salt, fresh black pepper to taste, dry mustard, and finely minced fresh spinach (if using) to the bowl.

Mix all ingredients thoroughly until well combined.

Add fine bread crumbs or wheat germ to the mixture and mix until it binds the ingredients together.

Form the lentil mixture into patties.

Heat a small amount of oil in a skillet over medium heat.

Cook the lentil-walnut burgers for about 5-7 minutes per side, or until golden brown and heated through.

Serve the lentil-walnut burgers on buns with your favorite toppings.