Heat oven to 350°F. In 10-inch skillet, melt 1 tablespoon butter over medium heat. Cook onion and bell pepper in butter 4 to 6 minutes, stirring occasionally, until crisp-tender.
Stir in flour, salt, mustard and pepper. Cook 1 to 2 minutes, stirring constantly, until smooth and well mixed; remove from heat.
Stir in milk. Heat to boiling, stirring constantly; boil and stir 1 minute. Stir in corn. Pour into ungreased 1-quart casserole.
In small bowl, mix cereal and 1 tablespoon butter; sprinkle over corn mixture. Bake uncovered 10 to 15 minutes or until mixture is bubbly and topping is golden.