Heat oven to 375°. Grease an oval 2 1⁄2-qt. casserole dish with 2 tbsp. butter. Heat remaining butter and the bacon in a 12" skillet over medium heat. Cook until bacon is slightly crisp, 10–12 minutes. Using a slotted spoon, transfer bacon to paper towels to drain.

Add onion to skillet; cook until slightly caramelized, 7–9 minutes. Add wine; bring to a boil. Reduce heat to medium; cook until reduced by half, 2–3 minutes. Stir in potatoes, salt, and pepper; cook until just tender, 8–10 minutes.

Spread half the potato mixture evenly in prepared dish; top with half each the reserved bacon and the cheese. Repeat with remaining potato mixture, bacon, and cheese; bake until top is browned and filling is bubbling, about 25 minutes.