Fry the Sausage. In a large skillet over medium-high heat, fry the sausage, breaking it up as it cooks, until browned and cooked through. Remove and set aside, leaving the drippings in the pan.

Make the Roux. Add the butter to the same skillet. Sauté the onion until softened, then stir in the flour and cook for 1 minute, stirring constantly, to make a roux.

Build the Cream. Slowly stir in the chicken stock, heavy cream, and whole milk. Simmer, stirring, until the sauce thickens enough to coat the back of a spoon. Season with the mushroom seasoning, salt, and black pepper, then stir in the grated Parmesan.

Finish and Use. Stir the sausage back into the cream. Use it right away in green bean casserole or creamed corn, or let it cool and refrigerate it for up to 3-4 days.