Quarter and core cabbage. Cut into wedges. Steam in a steamer at 200°F until very tender, about 30 mins.
Bring water and 1 tsp. of salt to a boil. Stir in rice, reduce heat to low and cook for 25 mins (rice will be underdone).
Cook beef in large pot crumbling with spoon or spatula, until browned, about 15 minutes. Drain. Stir in garlic powder, onion powder, pepper, and remaining 2TB salt. Stir in the rice and diced tomatoes.
Cut steamed cabbage into 1/2" strips and break apart.
Preheat convection oven to 325°F or conventional oven to 350°F.
Assemble the lasagnas in two 2" full hotel pans using the following amounts for each pan: spread 1 cup sauce on the bottom of pan. Top with 2.5 quarts cabbage, 2 cups meat mixture, 1 cup sauce, 2.5 quarts cabbage, 2 cups meat mixture, 1 cup sauce, 2.5 quarts cabbage, and 1 cup sauce.
Bake uncovered until sauce is bubbling and top is starting to brown, about 25 minutes.
