Preheat oven to 350-degrees F. Butter a 2-quart casserole dish; set aside.
Slice the sausage in half lengthwise, then cut across the halves making thick slices of half-rounds.
In a deep frying pan, over medium high heat, sauté the onions and potatoes in the 2 tablespoons of cooking oil or olive oil for about 3 minutes. Sprinkle with salt to taste while cooking.
Lower the heat to medium. Add the garlic, mushrooms, and sausage to the onions and potatoes. Sauté together, turning occasionally, until the mushrooms have reduced and given up much of their liquid. During the cooking process liquid from the mushrooms and onions will collect in the bottom of the frying pan. It will need to be cooked off or else the casserole will be too soupy. Stir the contents of the pan only as often as needed to keep things from sticking to the bottom of the pan, but not so often that the potatoes get mushy.
In a separate pan, melt the butter over medium heat. Add the flour and stir until smooth. Cook for 2-3 minutes, stir occasionally.
Pour the milk slowly into the pan, whisking quickly as you pour to prevent lumps. If there are some lumps, just keep stirring (whisking) until this sauce is as smooth as possible.
Cook and stir until the sauce thickens. It will be bubbly, but will not be boiling. Add salt and pepper to taste. Stir in the mustard. Taste and adjust salt and pepper as desired.
Pour sauce over the sausage and vegetable mixture in the other pan. Stir gently to combine well. Fold in frozen peas.
Pour into a casserole dish. Dot with butter if desired. Bake for 40 minutes at 350-degrees until hot and bubbly.