Halve peppers lengthwise, removing stem, seeds and membranes. Place peppers in boiling water for 3 minutes. Remove and sprinkle insides with salt.
In large skillet, cook meat, onion and garlic until meat is brown. Stir in undrained tomatoes, uncooked rice, worcestershire sauce, thyme, water, salt, pepper and garlic salt. Bring to boil;cover, reduce heat and simmer for 15 to 18 minutes, till rice is done. Make cornbread following package directions and add cheddar cheese.
Fill peppers with meat mixture. Place in 8x882-inch baking dish with any remaining meat mixture. Pour prepared cornbread mix over peppers. Bake, unovered, at 400*F for about 20 minutes or until cornbread is golden brown.