Baked Ziti without olive

1 tablespoon vegetable oil 1 cup chopped onion 1 cup chopped green bell pepper 1 cup chopped red bell pepper 1 1/2 cups chopped portobello, cremini, or button[...]

Salt and pepper 8 ounces hot Italian sausage, casings removed 2 tablespoons unsalted butter 2 tablespoons all-purpose flour 2 cups milk 8 ounces ziti 1 1/2[...]