In large, deep saucepan, heat butter until sizzling. Stir in flour. Cook over medium heat, stirring, 1 to 2 minutes or until flour is golden. Gradually whisk in milk and cook, stirring 8 to 10 minutes or until mixture is thick and smooth. Turn off heat, stir in salt, cumin, red pepper flakes, pepper, chiles and Cheddar. Stir until cheese melts. Add red peppers, dressing and cooked pasta, mixing well.
Pour into oval 8x10-inch lightly buttered casserole or similar baking dish. Bake for 20 to 25 minutes, until cheese is bubbly. Sprinkle with the sliced green onions.