Mashed Potatoes with cabbage

1 pound all-purpose potatoes (or red potatoes ) 1/8 cup milk 1/8 cup sour cream 3 tablespoon butter , divided 1/2 medium onion , chopped fine (about 1/3 cup)[...]

8 Tbsp. Unsalted butter 2 lrg Onions , thinly sliced 3 c. Cabbage thinly sliced 2 lb Russet potatoes , peeled, cut in 1" chunks 1/4 c. Half and half 2[...]