Step 1: Blanch the Green Beans. Preheat your oven to 350°F. Bring a large pot of salted water to a boil, add the green beans, and blanch for 3-4 minutes until bright green and tender-crisp. Drain and set aside.
Step 2: 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.
Step 3: 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.
Step 4: 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.
Step 5: Combine and Bake. Fold the green beans and fried sausage into the cream. Transfer everything to a baking dish and bake for 25 minutes, until hot and bubbling. Serve warm.