Place a rack in the center of your oven and preheat to 350°F. Grease a 9x13-inch baking dish with nonstick spray.
Thinly slice the cabbage: Peel off and discard tough outer leaves. Stand the cabbage up on its stem, then carefully slice in half form top to bottom. Place one half on the cutting board flat side up and use the point of your knife to remove the core of the cabbage. Flip the cabbage over so it’s flat side down and very thinly slice the cabbage into strips. Repeat with the second half.
Melt the butter in a large skillet over medium heat. Add the cabbage, onions, salt, and pepper and cook, stirring regularly, until softened, about 10 minutes. Add the garlic and nutmeg and cook for 1 minute, until fragrant.
Sprinkle the flour over the cabbage mixture and cook, stirring, until toasty golden, and dry, about 1 minute.
While stirring, slowly pour in the wine, then the broth, breaking up any clumps of flour and scraping browned bits off the bottom of the pan.
Increase the heat to a simmer. Let simmer 1 minute, then turn off the heat. Stir in the milk. Carefully taste and add more salt and pepper as desired.
Transfer the cabbage mixture to the prepared baking dish.
Make the topping: In a small bowl, melt the butter. Stir in the panko and salt. Sprinkle the topping over the casserole.
Bake for 20 to 25 minutes, until the top is golden-brown. Let cool for at least 5 minutes before serving.
Notes
TO STORE: Store leftover cabbage casserole in an airtight container in the refrigerator for up to 3 days. (Do not freeze leftovers; they won't thaw and reheat well.)
TO REHEAT: Reheat smaller portions in the microwave or reheat the entire casserole dish in the oven at 350ºF until warmed through.
TO MAKE AHEAD: Assemble up until the point of adding the topping, then bake the next day.