Lentils, brown rice, and mushrooms give these vegan meatballs flavor and substance. Serve with your favorite sauce and pasta.
Bring rice and enough water to cover by 3 inches to a boil in a medium saucepan. Reduce heat, season with 1 tablespoon salt, and simmer 20 minutes. Add lentils and continue to simmer until lentils are tender, about 27 minutes. Drain and let cool.
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.
With wet hands, form mixture into 21 compact meatballs, each 1 1/2-inch in diameter. Chill at least 1 hour and up to overnight.
Wipe out skillet; heat over medium high. Add remaining 3 tablespoons oil and meatballs. Cook turning frequently until browned all over, turning heat to medium if the pan starts to get too hot, about 10 minutes. Serve warm with pasta and sauce of your choice.