Warming Winter Chowder

Ingredients

• ["3 tablespoons olive oil (plus a bit more for serving)", "1 tablespoon butter", "1 yellow onion, finely diced", "2 shallots, finely diced", "2 leeks, white and light green parts only, chopped", "4 cloves garlic, smashed", "3 cups thinly sliced green cabbage (packed)", "2 carrots, chopped", "4 cups diced waxy potatoes (3/4\"-1\"" dice)""