Step 1

Place a rack in middle of oven; preheat to 375°. Cook 12 oz. bacon (about 12 slices), cut into ¼” pieces, in a medium skillet over medium heat, stirring often, until brown and crisp, 6–8 minutes. Using a slotted spoon, transfer bacon to paper towels to drain.

Step 2

Spoon 3 Tbsp. bacon fat into a medium saucepan; discard or reserve remaining fat for another use. Add 6 garlic cloves, finely grated, 3 cups heavy cream, 1 Tbsp. plus 2 tsp. Diamond Crystal or 1 Tbsp. Morton kosher salt, and 2 tsp. freshly ground pepper, set over medium heat, and cook, stirring occasionally, just until salt is dissolved, about 1 minute (it doesn’t need to be hot). Remove from heat.

Step 3

Peel 5 lb. russet potatoes (about 10 large) and, using mandoline, slice into about ¹⁄₁₆”-thick rounds. Pour a third of the cream mixture into a 13×9″ baking dish. Arrange potato slices in dish, fanning out a handful at a time so they overlap slightly and pressing down gently in between additions to compress, until all potatoes have been used and bottom of dish is covered. Pour remaining cream mixture over; press down on potatoes so they’re mostly submerged. Cover dish tightly with foil; bake until potatoes are tender, 60–70 minutes (a tester inserted in the middle should meet with barely any resistance). Remove foil, scatter 3 oz. sharp yellow cheddar, coarsely grated (about 1 cup), over, and continue to bake, uncovered, until cheese is melted, 6–8 minutes more.

Step 4

Let scalloped potatoes cool 5 minutes, then top with small dollops of ⅓ cup sour cream and scatter 4 scallions, dark green parts only, thinly sliced on a diagonal, and bacon over.

Do Ahead: Scalloped potatoes (without cheese and toppings) can be made 1 day ahead. Let cool; cover and chill. Reheat, covered, in a 350° oven 20 minutes, then uncover, top with cheese, and cook until melted, 7–9 minutes.