Preheat the oven to 350°F
Melt the butter in a large sauté pan over medium heat. Add the onions and cook, stirring occasionally to avoid Browning, until the onions are just starting to get a little bit translucent, about five minutes. Add the cabbage and cook, stirring occasionally to avoid frowning, for about 15 minutes, or until wilted. Add in the wine and continue cooking for about five more minutes, or until the wine is almost entirely reduced. Stir in the cream and mustard and season with salt and pepper to taste.
Transfer the mixture into a medium ovenproof pan, cover, and bake for 20 minutes.
Remove the pan from the oven and preheat the broiler. Check that the cabbage is tender and the sauce is nicely reduced; if it isn’t, put the pan over medium high heat and cook until the cabbage achieve the right tenderness and consistency.
Check the seasoning and adjust to taste. Spread the breadcrumbs evenly over the couch, drizzle them with the oil, and season with salt and pepper. Broil, uncovered, on the top rack for 1 to 2 minutes, or until the top is golden Brown. Watch closely so the breadcrumbs don’t burn. Serve immediately