If using fresh red sweet peppers, preheat oven to 450 degrees F. Cut peppers into quarters; remove stems and seeds. Place pepper quarters, skin sides up, on a foil-lined baking sheet. Bake 15 to 20 minutes or until skins are evenly charred. Bring edges of foil up around peppers to enclose. Let stand until cool enough to handle. Using a small sharp knife, peel off charred skin and discard. Chop the peppers. (If using roasted red sweet peppers, skip this step.)
Reduce oven temperature to 375 degrees F. Cook pasta according to package directions; drain.
In a small saucepan melt butter over medium heat. Add garlic; cook and stir 1 minute. Stir in flour, salt, and pepper until combined; cook and stir until bubbly but not brown. Whisk in milk all at once; cook and stir until thickened and bubbly. Cook and stir 1 minute more. Remove from heat.
In a large skillet cook sausage and onion over medium heat until sausage is no longer pink, stirring occasionally. Stir in vinegar and oregano. Stir in milk mixture. Add roasted red peppers, drained pasta, pasta sauce, and half of the cheese; stir until well combined. Transfer to a 3-quart baking dish. Cover with foil.
Bake 20 minutes. Uncover; sprinkle with remaining shredded cheese. Bake, uncovered, 5 minutes more or until cheese is melted and pasta is heated. If desired top with oregano leaves.