Prick potatoes with fork. Microwave on high 12 minutes or until fork-tender, turning once. Remove; cool until easy to handle. Preheat oven to 375 degrees F. Grease a 2-quart baking dish; set aside. In a large skillet cook leeks and 4 cloves of the garlic in melted butter over medium heat 5 minutes or until tender and browned, stirring occasionally.
Peel potatoes; place in large bowl. Mash with potato masher. Stir in leek mixture, sour cream, half of the cheese, 1/4 tsp. salt, and 1/4 tsp. black pepper. Stir in milk to make a creamy consistency. Spread in prepared dish. Bake 40 minutes or until starting to brown. Remove from oven. Make 6 indents in potato mixture. Place an egg in each indent. Bake 20 minutes more or until whites are set and yolks start to thicken. Sprinkle with remaining cheese. Cool slightly.
In bowl whisk oil, lemon juice, remaining 1 clove garlic, 1/4 tsp. salt and 1/4 tsp. black pepper. Add spinach and/or arugula; toss to coat. Serve on casserole.