• Preheat oven to 400°F. Grease 13- x 9-inch baking dish with olive oil; set aside. Use a mandoline or very sharp knife, cut potatoes and onion into 1/8-inch thick slices. Whisk together oil, broth, cream, garlic, thyme, salt and pepper.
• Arrange one-third of the potatoes in prepared dish, overlapping slices; top with half of the onions. Stir cream mixture and slowly pour one-third over top. Repeat layers, ending with remaining potatoes and cream mixture. Toss together Gruyère, Asiago and Parmesan; sprinkle over top. Arrange thyme sprigs over cheese. Cover with foil.
• Bake for about 1 hour or until potatoes are softened and sauce is bubbling. Remove foil; bake for 30 to 45 minutes or until potatoes are very tender and top is golden brown. Let stand for 30 minutes before serving.

Tip: Roasted garlic purée is available in the produce section, as well as the freezer section. Alternatively, roast a whole garlic head by cutting off the top third, then placing garlic head on a small sheet of foil; drizzle with 1 tsp olive oil. Wrap tightly; bake in 400°F oven for about 45 minutes or until very tender and golden brown.