Meanwhile, pulse mushrooms and onion in food processor until minced. Heat a large skillet over medium-high. Add 1 tablespoon oil and mushroom mixture. Cook stirring occasionally until mushrooms have released most of their moisture and become golden around the edges of the pan, about 4 minutes. Transfer to a large bowl; stir in flaxseed. Let sit at least 5 minutes. Mix in lentils, brown rice, breadcrumbs, and parsley. Stir, pressing mixture against side of bowl with spoon to break up some of the lentils until the mixture clumps together. Season with salt and pepper.