Wash and de-stem the baby portabella mushrooms, then place them into a food processor/blender and pulse until they are diced (not blended). Empty them into a large bowl along with the drained and rinsed black beans and lentils. Add the diced red onion to the bowl. With your hands, mash the lentils and black beans until they are no longer whole, but not too mushy. Add the tomato paste, Worcestershire sauce, Parmesan cheese, Italian seasoning, flaxseed meal, almond flour, cassava flour, fennel seeds, garlic powder, salt, and pepper. Using a spoon, stir to combine.
Spoon a heaping tablespoon of the meatball mixture and gently roll between your hands to form a ball. Place onto a parchment-lined baking sheet and repeat. Bake for 30 minutes, flip them, and then bake an additional 10 minutes.