Adjust oven rack to upper middle position and heat oven to 400 degrees. Grease 13x9 inch baking dish. Combine cheese, 3 tbs oil, panko, & 1/2 tsp pepper in bowl; set aside.
Heat 2 tbs oil in 12 inch skillet over medium heat until shimmering. Additions, 1/2 tsp salt, & 1.4 tsp pepper; cook, stirring frequently, until browned, about 15 minutes. Add garlic & 1/2 tsp thyme; cook until fragrant, about 30 seconds. Add 1/4 cup broth & cook until nearly evaporated, scraping up any browned bits, about 2 minutes. Remove from heat; set aside.
Toss potatoes, remaining 3 tbs oil, 1 tsp salt, 1/2 tsp pepper, & remaining 1/2 tsp thyme together in bowl. Arrange half of potatoes in prepared dish, spread onion mixture in even layer over potatoes, & distribute remaining potatoes over onions. Pour remaining 3/4 cup broth over potatoes. Cover dish tightly with foil and bake for 1 hour.
Remove foil, top gratin with reserved cheese mixture, & continue to bake until golden brown and potatoes are completely tender, 15-20 minutes. Let cool for 15 minutes.