Core the cabbage and put in a large microwave-proof bowl. Add 1/3 cup water and cover with plastic wrap. Microwave on high for 20 minutes, remove soft leaves and continue microwaving until all the usable leaves are soft. Stack the leaves as you remove them and cover.
Mix the filling ingredients in a separate bowl and get your large casserole dish out. (You may need another small casserole dish).
Place filling near the base of each cabbage leaf and roll up, placing seam side down in the casserole. Crowd the rolls as much as possible. Continue until you are out of filling.
Spoon the tomato soup over the rolls evenly.
Bake at 300-325 degrees for at least 2 hours, covering when they start to brown.
I've guessed at the serving sizes since Grandma didn't tell me! And cabbage rolls do take a long time but are worth it all.