In medium skillet, sauté 3 tablespoons butter with onions and peppers. When onions become golden, add steak and tomatoes. Cook one to two minutes more, remove from heat. Set aside.
Cook pasta according to package directions. Drain well; set aside.
Meanwhile, melt remaining 3 tablespoons butter over medium heat in medium saucepan. Using whisk, stir in flour, salt and garlic; cook 2 to 3 minutes, stirring constantly, until mixture is lightly golden. Slowly add milk and continue stirring until mixture comes to boil. Reduce heat to low and add shredded cheeses; stir until mixture is smooth. Pour over prepared pasta.
Gently stir in prepared vegetables and meat; serve hot.