• Cook potatoes in large saucepan of boiling salted water until just tender. Drain and cool. Cut potatoes into 1/3-inch-thick slices, removing any loose peel. Set aside.
• Combine onions and 1/4 cup water in a large nonstick skillet. Cover and simmer over medium heat until onions are tender, stirring occasionally and adding more water by tablespoonfuls if mixture is dry, about 12 minutes. Increase heat to medium-high; add 2 tablespoons oil, then mushrooms; saute uncovered until onions are deep golden, about 10 minutes. Add spinach, garlic, thyme, and nutmeg; stir 3 minutes. Remove from heat. Stir in sour cream and milk. Season with salt and pepper.
• Preheat oven to 350F. Lightly oil 11x7x2-inch glass baking dish. layer half of potato slices in prepared dish, overlapping slightly. Sprinkle with salt and pepper. Spread half of onion and spinach mixture over potatoes. repeat layering with remaining potatoes then onion and spinach mixture. Top with cheese. Bake gratin until hot, about 30 minutes. Let stand 5 minutes and serve.
