- In a frying pan, melt 2 tablespoons of butter. Cook onion over medium heat until tender. Remove from frying pan and set aside.
- In a large pot, bring three quarts of salted water to a boil. Add the diced potatoes and return to a boil. Reduce heat and parboil for 5 minutes. Drain in a colander and set aside.
- In a saucepan, melt 1/4 cup of butter over medium low heat and, while stirring constantly, slowly add the flour. Cook the roux until it starts to brown. Add milk, mustard, salt and pepper. Continue heating and stirring until sauce thickens, 10 to 15 minutes.
- Add reserved onions, peas and cheese, stirring until well incorporated and the mixture smooth and uniform. Do not boil or the cheese will separate. Add drained potatoes and fold in to mix.
- Prepare a 2-quart casserole dish with a light coating of vegetable spray. Add the cheese and potato mixture. Sprinkle with additional ground black pepper. Sprinkle with the crumbled bacon.
- Cover with a lid or aluminum foil and bake in a preheated 375°F oven for 20 minutes. Remove cover. Bake an additional 10 to 20 minutes or until potatoes are tender. If the cheese browns too fast, recover with the lid of foil.