Mashed Turnips and Potatoes with Horseradish Bread Crumbs

Ingredients

2 pounds yellow-fleshed potatoes such as Yukon Gold

1 1/4 lb turnips

2 1/4 teaspoons salt

7 tablespoons unsalted butter, cut into tablespoons

1 cup coarse fresh bread crumbs (from 2 slices firm white sandwich bread)

2 tablespoons drained bottled horseradish, patted very dry between paper towels

3/4 cup whole milk

1/2 cup thinly sliced scallion greens