Preheat oven to 350 degrees. In two small pots, cook rice and lentils to package directions.
While those are cooking, wash and dry all produce. Chop garlic and scallions (separating the white and green parts). Cut out the core of the cabbage and throw away. Chop remaining cabbage into small chunks.
In a large pan over medium high heat, add oil and ground beef. Cook until no longer pink, about 7-8 minutes. Put cooked meat in clean, large bowl.
In the same pan you cooked the meat in, add the scallion whites and garlic. Cook over medium high heat until soft, about 2 minutes. Add cabbage, sage and red pepper flakes. Cook until soft, about 5-7 minutes. While cabbage cooks, mix the rice and lentils in the bowl with the meat.
In a casserole dish, pour half the beef mixture in an even layer, then top with half the cabbage.
Pour half the crushed tomatoes over cabbage. Repeat the three layers one more time. Bake until casserole is hot and bubbly, about 30 minutes. Top with scallion greens just before serving.