Blanch cabbage for 15 minutes (or use leftover cooked cabbage). Shred. Meanwhile, cook 3 slices of bacon until crispy.
Butter a 2-quart casserole. Preheat oven to 350 degrees.
In a small saucepan over medium-low heat, melt butter, then whisk in flour. Stir continuously until thick and bubbly. Whisk in heavy cream and bring to a gentle simmer; stirring continuously for 1-2 minutes. Sauce will be thick, add a small amount of cream if too thick. Season with salt & pepper to taste.
Layer half of the cabbage in prepared casserole. Sprinkle with half of the crumbled bacon and half of the Gruyere cheese. Spoon half of the white sauce over top; repeat with remaining cabbage, Gruyere and white sauce.
Combine panko and parmesan cheese and spread evenly on top. Bake for 30 minutes or until golden brown and bubbly. Allow to rest 10 minutes before serving.