4 tbsp. unsalted butter, plus more for greasing
3 cloves garlic, finely chopped
1 medium yellow onion, thinly sliced
¼ cup flour
1½ cups heavy cream
½ cup milk
3 tbsp. finely chopped thyme
Kosher salt and freshly ground black pepper, to taste
2½ lb. Yukon gold potatoes, peeled and very thinly sliced
¼ tsp. paprika