Cheesy Spaghetti Squash Casserole

Name: Cheesy Spaghetti Squash Casserole
Ingredients: 1 tablespoon olive oil plus more for the baking dish; 1 spaghetti squash , about 3 pounds; 2 teaspoons kosher salt; 1/2 teaspoon freshly ground black pepper; 1 onion , finely chopped; 1 clove garlic , minced; 1/2 cup jarred roasted red peppers , thinly sliced; 2 tablespoons chopped fresh sage leaves; 3 large eggs; 1 cup whole milk; 3 tablespoons all-purpose flour; 1 1/2 cups (5 ounces) grated Jarlsberg or baby Swiss cheese; 1/2 cup (2 ounces) finely grated Parmesan