Autumn Turkey-Stuffed Cabbage

1 head cabbage
1/2 lb lean ground beef
1/2 lb ground turkey
2 small onions, one minced, one sliced
1 slice stale whole wheat bread, crumbled
1/4 C water
1/8 tsp black pepper
1 can (16 oz) diced tomatoes
1 C water
1 medium carrot, sliced
1 Tbsp lemon juice
2 Tbsp brown sugar
1 Tbsp cornstarch