Place onion in bottom of a 4-quart electric slow cooker; add beef roast. Sprinkle 2 tablespoons flour, oregano, salt, and crushed red pepper over roast. Add green pepper and next 3 ingredients to slow cooker. Cover and cook on low setting 8 hours or until roast is tender.
When roast is done, cook spaghetti according to package directions, omitting salt and fat; drain well.
While pasta cooks, remove roast from cooker, and slice diagonally across grain; keep warm. Combine remaining 2 tablespoons flour and water, stirring until smooth; add to sauce in cooker. Cook, uncovered, 10 to 15 minutes or until mixture is thickened. Serve meat and sauce over spaghetti.
Sizes of slow cookers vary. Cut the roast in pieces to fit the shape of your slow cooker, if necessary.
Cooking Light Low-Fat, Low-Calorie Quick and Easy Cookbook