1. In a large saucepan or stock pot, re-heat reserved bacon fat over medium heat, add butter.
2. Add onion and sauté for 5-7 minutes until translucent and tender.
3. Add flour and stir for 1 minute, to cook raw flour taste out. Whisk in half and half.
4. Reduce heat to medium low and simmer until mixture is reduced by one fourth and has thickened.
5. Stir or whisk in 1 2/3 cup of Monterey Jack and cheddar cheeses, and Parmesan cheese until melted, creamy and thickened. Add mustard powder. Season to taste with salt and pepper, mix well.
6. Stir in corn, cook 1 minute more. Remove from heat and gently stir in cooked and drained pasta and 3/4 of the chopped bacon.
7. Pour mixture into greased 9x13 baking dish. Cover with remaining cheese.
8. Bake at 350° F for 20 minutes, or until cheese is bubbly and slightly golden brown. Serve warm, garnish with remaining bacon and chopped green onions.