Rub inside of 2-1/2 to 3-qt. gratin dish with garlic clove, coat with the 1 Tbsp. butter.
With a mandoline or very sharp knife, slice onion about 1/16-inch thickness. In a skillet over low heat cook onion in olive oil until tender, sprinkling well with salt and pepper. Remove from heat; stir in thyme leaves. Set aside.
In a small saucepan heat milk and cream just until simmering. In a saucepan melt butter over medium heat and whisk in hot milk and cream a little at a time until incorporated. Return to heat; bring to a gentle boil. Cook 3 to 5 minutes or until thickened.
Peel all potatoes. Slice to about 1/16-inch thickness. Layer one-third of russet and sweet potato slices in prepared dish. Season with salt, ground pepper, and two or three light gratings of nutmeg.
Scatter about one-third of onions on potatoes. Spoon one-third of cream mixture on potatoes. Create two more layers with remaining potatoes, seasonings, onions, and cream mixture, finishing with cream mixture on top.
Bake, uncovered, in a preheated 350º oven for 45 minutes. Increase oven temp to 425º. Bake 10 to 15 minutes more or until bubbly, golden crusty brown and potatoes are tender. Remove from oven. Let stand 10 minutes.