Heat oven to 400°F. In 2-quart saucepan, melt butter over medium heat. Cook onion in butter 3 minutes, stirring occasionally, until tender. Add flour; cook 1 minute, stirring constantly. Stir in whipping cream, salt and pepper. Cook 30 seconds or until thickened. Remove from heat; stir in spinach, 1/2 cup of the Parmesan cheese and the Monterey Jack cheese until blended.