Look for xanthan gum with the gluten-free baking mixes and flours; it's available in most supermarkets these days. You can get semiclose results with cornstarch, but the spuds won't be quite as creamy.
3 pounds baking potatoes, peeled and cubed
1/2 cup fat-free milk
1/2 cup powdered nonfat milk
1 cup unsalted chicken stock
3 tablespoons unsalted butter
1/2 teaspoon xanthan gum
1 teaspoon kosher salt
1/2 teaspoon freshly ground black pepper
1 large garlic clove, grated on a microplane grater
Place potatoes in a Dutch oven. Cover with cool water to 2 inches above potatoes. Bring to a boil; reduce heat, and simmer 15 minutes or until very tender. Drain.
Pour fat-free milk into Dutch oven; gradually whisk in powdered milk. Heat over medium heat until warm (about 2 minutes), stirring constantly with a whisk until powder dissolves and mixture is slightly thickened. Remove from heat.
Press potatoes through a ricer into pot with milk mixture. Cover and keep warm. (There's no need to stir together yet; you can leave it as is.)
Combine stock and butter in a small saucepan; cook over medium-high heat until mixture simmers and butter melts. Remove saucepan from heat; whisk in xanthan gum until mixture is thick and emulsified.
Pour stock mixture over potato mixture; sprinkle with salt, pepper, and garlic. Stir well to combine.
Squeeze thawed spinach superdry in a clean kitchen towel. Stir spinach and crumbled feta cheese into potatoes. Serve soon for creamiest results.