Preheat oven to 425.
Trim the cabbage bottom, so cabbage rests flat. Holding the cabbage firmly, carve a circle out of the top of the cabbage (just as you would carve a pumpkin). Remove "top" of cabbage and set aside.
Drizzle cabbage "top" and "bottom" with 1 Tbsp olive oil and rub the olive all over the cabbage. Sprinkle with garlic salt and pepper.
Toss beet (or carrot) slices with remaining 1 Tbsp olive oil, garlic, garlic salt and pepper.
Place cabbage "top" and "bottom" on baking sheet and place into oven.
After 15 minutes, flip cabbage and roast another 15 minutes. Add beets (or carrots) to the baking sheet. Flip cabbage again and roast an additional 15-30minutes, or until outer cabbage leaves begin to crisp and the full cabbage is tender and the beets (or carrots) are tender and golden. Cooking time may vary, depending on size of cabbage.
Once very tender, remove cabbage from oven. Place cabbage carved side up on a serving dish. Fill with golden beet (or carrot) coins.
Slice cabbage and top each slice with a small pat of salted butter.
Serve.