Mashed Potatoes with Garlic, Mascarpone, and Caramelized Leeks

3 T butter
4 medium leeks (white and pale green parts only), halved lengthwise, rinsed, chopped (about 5 cups)

1/2 cup (or more) whole milk
12 garlic cloves, coarsely chopped (about 1/2 cup)

3 3/4 lbs. russet potatoes, peeled, cut into 1-inch pieces
3/4 cup mascarpone cheese (about 6 ounces)

Melt butter in large nonstick skillet over medium heat. Add leeks and stir to coat. Cover and cook until leeks are golden brown, stirring every 5 minutes, about 20 minutes total. Season to taste with salt and pepper. (Can be prepared 1 day ahead. Cover and refrigerate. Rewarm over medium heat before using.)

Combine 1/2 cup milk and garlic in medium saucepan. Bring to simmer over medium-low heat. Reduce heat to very low, cover, and cook until garlic is soft, about 10 minutes. Remove from heat. Using back of fork, mash milk-garlic mixture to paste. Set aside.

Cook potatoes in large pot of boiling salted water until tender, about 25 minutes. Drain well. Return potatoes to same pot; mash until almost smooth. Add mascarpone and milk-garlic mixture; mash until smooth. Season with salt and pepper. (Can be made 2 hours ahead. Let stand uncovered at room temperature. Rewarm over medium-low heat, stirring and thinning with milk by 1/4 cupfuls if dry.) Top with leeks.

*Italian cream cheese; sold at many supermarkets and at Italian markets. If unavailable, blend 3/4 cup cream cheese with 3 tablespoons whipping cream and 1 1/2 tablespoons sour cream.