French Onion Mashed Potatoes

Directions

• Preheat oven to 400°F. Butter a 2-quart baking dish and set aside.
• Heat 2 tablespoons butter in a large skillet over medium-low heat. Add onions, season with salt and pepper and cook, stirring occasionally, until onions are deeply golden and very soft, about 30 minutes. Stir in thyme and set aside.
• Place potatoes in a large pot and cover with an inch of cold water. Bring to a boil, season generously with salt, then cover and reduce to a simmer. Cook until fork tender, about 15 minutes.
• Meanwhile, gently heat milk, remaining 4 tablespoons garlic butter, 1 teaspoon kosher salt and ¼ teaspoon black pepper in a small saucepan. Remove from heat and whisk in sour cream.
• Drain potatoes in a colander and return to the warm pot. Let sit a few minutes to dry out. Pass potatoes through a ricer into a large bowl, then mix in warm cream mixture. Stir in 1½ cups shredded gruyere and half the caramelized onions; season with salt and black pepper.
• Transfer to prepared baking dish and spread in an even layer. Sprinkle with remaining caramelized onions, remaining ½ cup gruyere and grated parmesan. Bake until warmed through, about 20 minutes. Broil for 2-3 minutes, or until top is golden brown. Serve hot.