3 tablespoons butter
4 medium leeks (white and pale green parts only), halved lengthwise, rinsed, chopped (about 5 cups)
1/2 cup (or more) cream
3 tablespoons pureed fresh garlic (about 12 garlic cloves)
3 3/4 pounds russet potatoes, peeled, cut into 1-inch pieces
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 cream and garlic in medium saucepan. Bring to simmer over medium-low heat. Reduce heat to very low, cover, and cook for 3 minutes. Remove from heat. 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 garlic cream 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.
Makes 8 servings.