Heat oven to 425. Grease 13"x9" inch pan. In large skillet, brown sausage and onions. Remove from skillet and drain on paper towels. Arrange sausage mixture in greased pan. Roast red peppers by broiling 5 minutes until skin blackens. Place in plastic bag to steam for 10 minutes. Peel skin from peppers and chop. Sprinkle half of the red peppers over the sausage mixture. Top with spinach.
In large bowl, combine flour, parmesan cheese, salt and basil. In another large bowl, combine eggs and milk. Beat until smooth. Add egg mixture to flour mixture. Beat until well blended. Pour over spinach**.
Bake at 425 for 20 to 25 minutes or until knife inserted in center comes out clean. Sprinkle casserole with remaining red pepper and provolone cheese. Bake additional 2 minutes or until cheese is melted. Let stand 5 minutes. Cut into squares.
**Casserole can be covered and refrigerated for up to 2 hours.