Heat oven to 375°F. Grease a 9x13-inch baking dish.
Boil a large pot of salted water. Blanch cabbage 3–4 minutes until just tender.
Drain well, then press out excess water with towels. Set cabbage aside.
Melt 1 tbsp butter. Toss with breadcrumbs and half the Parmesan. Set aside.
Melt remaining 3 tbsp butter in a saucepan. Soften onion 3 minutes over medium heat.
Stir in garlic for 30 seconds. Sprinkle in flour and cook, stirring, 1 minute.
Whisk in warm milk gradually. Simmer, whisking, until thick and smooth, 3–5 minutes.
Season with mustard, nutmeg, salt, pepper, and red pepper flakes if using.
Off heat, stir in most Gruyère until melted. Reserve a small handful for topping.
Combine cabbage with cheese sauce. Spread in dish. Top with reserved cheeses and breadcrumbs.
Bake 20–25 minutes until bubbling and golden. Broil 1 minute if needed. Rest 5 minutes.
Scatter parsley and serve warm.
Notes
Variation: Swap half the Gruyère for sharp cheddar, or add crisp bacon bits to the topping. For a lighter sauce, use 1 cup milk and 1 cup low-sodium chicken broth. Storage: Refrigerate leftovers up to 3 days; reheat covered at 350°F until hot.This recipe is an original creation inspired by classic Easy Cabbage au Gratin Recipes flavors. All ingredient ratios and instructions are independently developed.