Prep and heat the oven. Preheat to 375°F (190°C). Lightly grease a 9x13-inch baking dish or a similar 3-quart casserole.
Blanch the cabbage. Bring a large pot of salted water to a boil.
Add the cabbage and cook for 3–4 minutes until just tender. Drain well, then spread on a towel-lined tray to remove excess moisture.
Make the sauce base. In a medium saucepan, melt 2 tablespoons butter over medium heat. Whisk in the flour and cook for 1 minute, stirring, to remove the raw taste.
Add liquids. Slowly whisk in the warm milk, then the broth.
Simmer, whisking, until thick and smooth, about 3–4 minutes.
Season and enrich. Stir in salt, pepper, garlic powder, onion powder, and Dijon if using. Off the heat, stir in 1/2 cup cheddar and the Parmesan until melted.
Combine. In a large bowl, toss the drained cabbage with the sauce. Transfer to the prepared baking dish and spread evenly.
Top with cheese and crumbs. Mix the remaining 1/2 cup cheddar with the breadcrumbs.
Drizzle in the melted butter and toss. Sprinkle evenly over the casserole.
Bake. Bake for 25–30 minutes until bubbling at the edges and the top is golden brown. If needed, broil for 1–2 minutes to deepen the color—watch closely.
Rest and serve. Let the casserole rest for 10 minutes before scooping.
Garnish with parsley if you like.