Trim as much of the leaves from the cabbage cores as possible.
Chop the cores into rough 1 or 2 inch piece,s so they steam evenly.
Steam them for an hour (I used my Instant Pot, but you can use a steamer basket inside a stockpot).
Let cool, then add to a food processor.
Blend until smooth, up to 10 minutes.
Heat up olive oil over medium heat.
Mince garlic and sage and saute 5 minutes, or until garlic starts to brown.
Mix into mash. Heat as desired (in an oven or on stove top).
If using dairy, mix into mash.
Add salt and fresh ground black pepper to taste. Serve warm.