In a 10-inch skillet cook sausages for 15 minutes or until no longer pink, turning occasionally. Add sweet peppers and onion to skillet. Cook for 5 minutes more or until vegetables are tender, stirring occasionally.
Meanwhile, in a large pot cook spaghetti, with crushed red pepper added to water according to package directions. Drain pasta; return to pan.
Add sausage mixture to spaghetti in pan. Stir in chicken broth. Add spinach; toss just until combined and spinach is slightly wilted. Sprinkle each serving with Asiago cheese and, if desired, additional crushed red pepper.