Have ready a greased 9x13" casserole dish. Add the oil to a large saute pan over medium-high heat. Add the onion and saute 5 minutes. Add sausage and break apart with a wooden spoon. Saute for an additional 7-10 minutes. Remove from the heat and set aside to cool. Remove the crust from the bread and cut, or tear, into 1" cubes. Set aside until ready to use.

For the custard, whisk together the eggs, half and half, pepper and salt, making sure all of the ingredients are evenly combined and no eggs remain intact. Line the casserole dish with about ⅔'s of the bread cubes, forming a single, even layer. Scatter the sausage and onion mixture evenly across the layer of bread cubes. Sprinkle with half of the cheese and all of the parsley. Pour the custard evenly over the casserole and top with the remaining bread cubes.

Press down gently until the custard runs up and over the bread cubes. Cover with plastic wrap and refrigerate for at least four hours, but preferably overnight. Set out the casserole for at least one hour before baking, allowing it to come as close to room temperature as possible. Preheat the oven to 325°.