Preheat oven to 350°; spray 2 1/2 quart baking dish with cooking spray. In large saucepot, heat potatoes and enough salted water to cover by about 2 inches to a boil over high heat; reduce heat to medium and simmer 15 minutes or until fork-tender. Drain potatoes; return to saucepot. Add sour cream, butter, nutmeg, salt and pepper; with potato masher, mash potatoes until slightly chunky.
In large skillet, cook bacon over medium-high heat 8 minutes or until crisp, stirring occasionally; with slotted spoon, transfer to paper towel-lined plate.
In same skillet with drippings, cook leeks over medium heat 8 minutes or until tender, stirring occasionally. Add garlic; cook 1 minute or until fragrant, stirring frequently.
Fold 1 cup cheese, bacon and leek mixture into potato mixture; spread in prepared dish and bake 20 minutes. Sprinkle remaining 1 cup cheese over casserole; bake 10 minutes or until top is lightly browned. Makes about 8 cups.