Potato, Spinach, and Sausage Casserole | America's Test Kitchen

Ingredients

• 3 tablespoons unsalted butter , plus extra for greasing baking dish
• 4 teaspoons olive oil
• 1 (16-ounce) bag fresh spinach , washed, dried, and stemmed
• 1 pound sausage , links cut in half lengthwise and then crosswise into 1/4-inch-thick pieces
• 2 large red onions , halved and sliced thin
• 1 medium clove garlic , minced
• 3 pounds russet potatoes (about 6 medium), peeled and cut into 1-inch chunks
• 1/2 cup heavy cream
• 1/4 cup low-sodium chicken broth
• 2 tablespoons cider vinegar
• 1/8 teaspoon nutmeg
• 3/4 teaspoon table salt
• 1/2 teaspoon ground black pepper
• 2 cups shredded Gruyère or Swiss cheese