Preheat oven to 350 degrees F. Spray a 9x9 baking dish.

Bring a large pot of salted water to a boil. Add cabbage and cook until slightly tender, about 10 minutes. Drain cabbage completely. Let it sit in the colander for a few minutes to ensure it is not watery. Add cabbage into baking dish.

Melt butter in a medium-sized saucepan over medium high heat. Add onion and sauté until slightly browned, about 3 minutes. Add in flour and mix. Slowly pour in the milk, continuing to whisk to combine. Add salt and pepper, and continue mixing. Add in cheese, a little bit at a time, and continue whisking until sauce becomes thickened.

Pour cheese sauce on top of cabbage and mix. Sprinkle bread crumbs on top. Bake for 30 minutes, or until the top is browned.