Cook all potatoes, 1/2 of the minced garlic, and bay leaf in large saucepan of boiling salted water until very tender, about 12 minutes. Drain; discard bay leaf. Return potatoes to pot. Mash until smooth; cool slightly. Mix in 3 tablespoons cheese. Season with salt and pepper. Whisk in eggs.
Wash and remove portabello mushroom stems and chop finely. Heat 2 tablespoons butter in large skillet over medium-high heat. Add mushroom stems, remainder of the minced garlic and onion; saute 5 minutes. Stir in parsley and rosemary (if you cannot find fresh rosemary, substitute 1/4 teaspoon dried rosemary). Season with salt and pepper.
Preheat oven to 400°F. Brush rounded sides of mushrooms with 2 teaspoons olive oil. Place on baking sheet, gill sides up. Bake 10 minutes. Reduce oven temperature to 350°F. Divide onion mixture among mushroom caps. Mound potato mixture in caps. Sprinkle generously with cheese. Bake until heated through, about 25 minutes.
Heat 1 tablespoon of Organic Valley Butter in large pot over medium heat. Add spinach; saute 2 minutes. Season with salt and pepper. Divide among 4 plates. Top each serving with 1 stuffed mushroom.