Preheat oven to 425°F. Heat oil in a large saucepan over medium-high heat. Add onion and mushrooms; cook about 3 minutes, stirring frequently, until onions are softened. Add stock and quinoa, scrape up any browned bits from the bottom of the pan, and bring to a boil. Cover; simmer over low heat about 15 minutes until quinoa is popped and tender. Uncover quinoa and continue cooking 3 to 4 minute, stirring occasionally, until no liquid remains.

Meanwhile, smash beans in a large bowl with a potato masher until almost smooth; add bread crumbs, parsley and mustard. Add cooked quinoa and mix until evenly combined. Taste and season with salt and pepper as needed. Line a baking sheet with parchment paper and generously coat with cooking spray.

Drop quinoa mixture in 6 portions (about 2/3 cup each) onto the baking sheet, spacing them evenly apart. With damp hands, pat each portion into a 4-inch wide, 1/2-inch thick patty. Generously spray patties with cooking spray. Bake 20 to 25 minutes, until golden brown on the bottom. Carefully flip burgers with a spatula. Bake 10 to 15 minutes more. Serve in buns or lettuce with toppings, as desired.