Preheat the oven to 350ºF. Coat a large baking sheet with cooking spray or a Silpat.
In the bowl of a food processor, combine beans, roasted red peppers, and onions. Pulse until chopped but not smoothly pureed.
Transfer the mixture to a medium-sized bowl and stir in garlic, parsley, oregano, egg, breadcrumbs, salt, and pepper until thoroughly combined.
Scoop out a rounded tablespoon of the bean mixture and form it into a ball by rolling it between the palms of your hands. Place the "meatballs" on the prepared baking sheet, spacing evenly. Repeat with the remaining bean mixture.
Bake the balls until they are firm to the touch and have become lightly golden brown, 15 to 20 minutes.
In a large saucepan, heat marinara sauce over medium heat until simmering. Add the "meatballs" and stir to coat. Simmer until the sauce thickens slightly, stirring occasionally, 10 to 15 minutes.
Meanwhile, bring a large pot of water to a boil and cook spaghetti according to package directions. Serve cooked spaghetti topped with sauce, "meatballs," and freshly grated Parmesan cheese.