Preheat oven to 350°F. Grease a 13-by-9-inch baking dish.
In a skillet over medium heat, sauté the onion and pepper in the cooking oil until softened, about 3-4 minutes. Remove from heat and set aside.
Bring 12 c. water to boil in a large pot. Add the macaroni and cook until tender but not mushy, about 8 minutes.
Drain the macaroni and return to the hot pot. Stir in the sautéed onions and peppers, cheddar cheese, evaporated milk, mayonnaise or butter and Parmesan cheese. Season with salt and pepper and mix until the cheese starts to melt.
Transfer the mixture to your greased baking dish. You can sprinkle extra cheese on top if you want.
Bake 20-25 minutes or until the cheese has melted and the top is golden brown.
Remove from the oven and let it sit for 10 minutes before serving. Mmm … cheesy goodness!