Pre-heat the oven to 350 ̊F.

Heat a large pot of water to a boil. Add the pasta and cook according to the package directions, adding the green peas to the pot for the last 2 minutes of cooking. Drain the pasta and set aside.

In the meantime, heat a 2- to 3-quart saucepan over medium heat. Add the bacon and cook to render out the fat – about 6 to 8 minutes. When the bacon is almost crispy, remove the bacon from the pan and set aside. Add the onion to the pan and cook until translucent and very tender, but not brown. Add the flour and mustard powder and stir, cooking for 2 minutes. Whisk in the milk, stirring constantly to avoid lumps. Stirring regularly, bring the sauce to a boil. Once it has boiled and thickened, remove the pan from the heat and stir in the grated cheeses, salt and pepper.

In a large mixing bowl, combine the cooked pasta and peas, cooked bacon, cherry tomatoes and cheese sauce. Fold everything together and transfer to a casserole or baking dish. Combine the topping ingredients and spread the mixture on top of the pasta. Cook in a 350 ̊F oven for 30 to 40 minutes.