Preheat oven to 350 degrees.  In a 9x13 pan spread one package of the refrigerated croissants into the bottom of the pan.  Bake croissants for 7-9 minutes or until they are nice and golden.  Remove from oven when baked. 

Meanwhile, brown ground beef and onions in a skillet. Once cooked through, drain the fat from the skillet.

Add the chopped cabbage to the skillet filled with meat and onions and let the cabbage cook/steam for a few minutes.

Once the Runza filling is cooked, layer it on top of the baked croissants in the pan.  Sprinkle the Mozzarella cheese over the meat mixture.

Carefully top the casserole with the second package of refrigerated croissants.

Bake uncovered for 30 minutes at 350 degrees.  Remove from oven and cover pan with foil and bake for another 10 minutes - this allows the casserole topping to be a bit softer.

Cut into squares and serve and enjoy hot.