Preheat oven to 350 degrees F (175 degrees C). Grease a 3-quart baking dish.

Melt butter in a large saucepan over medium-high heat. Saute onion, garlic, and thyme in hot butter until onion is tender, 3 to 5 minutes. Whisk flour, salt, and black pepper into onion mixture.

Gradually whisk milk and cream into onion mixture; bring to a boil, reduce heat to low, and simmer until sauce thickens, about 2 minutes.

Stir sour cream into sauce until smooth; add fontina cheese and stir until cheese melts.

Arrange 1/2 the potatoes in the bottom of the prepared baking dish. Pour 1/2 the cheese sauce over the potatoes. Repeat with remaining potatoes and cheese sauce. Top with bread crumbs to cover potatoes entirely.

Bake in the preheated oven until potatoes are tender and bread crumbs are golden brown, 60 to 70 minutes.