Make Ahead Mashed Potatoes

I love these mashed potatoes ~ they taste delicious and can be made up to 8 hours ahead and just popped back into the oven for heating closer to meal time. 1/4 cup butter 6 Tablespoons butter 2 1/4 pounds white onion, thinly sliced 3 Tablespoons balsamic vinegar 2 teaspoons fresh thyme, chopped 3 1/2 pounds russet potatoes, cut in 1" cubes 6 Tablespoons milk, more as needed 10 cloves roasted garlic Melt 1/4 cup butter in large skillet over medium heat. Add onions; sauté until deep golden, about 30 minutes. Add vinegar and thyme. Reduce heat to low; sauté 4 minutes. Season with salt and pepper. Meanwhile, cook potatoes in large pot of boiling salted water until very tender, about 20 minutes. Drain. Return to same pot; mash until smooth. Add 6 tablespoons milk, garlic and 6 tablespoons butter; stir to blend. Thin with 2 tablespoons milk if necessary. Season with salt and pepper. Transfer potatoes to serving dish. Spoon onion mixture atop potatoes. (Can be prepared 8 hours ahead. Cover with foil and refrigerate. Rewarm in 350°F oven about 30 minutes.) Serve hot.