Step 1
Place a rack in center of oven; preheat to 375°. Cook dinner 12 oz. bacon (about 12 slices), reduce into ¼” items, in a medium skillet over medium warmth, stirring typically, till brown and crisp, 6–8 minutes. Utilizing a slotted spoon, switch bacon to paper towels to empty.
Step 2
Spoon 3 Tbsp. bacon fats right into a medium saucepan; discard or reserve remaining fats for one more 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 floor pepper, set over medium warmth, and cook dinner, stirring often, simply till salt is dissolved, about 1 minute (it doesn’t have to be scorching). Take away from warmth.
Step 3
Peel 5 lb. russet potatoes (about 10 giant) and, utilizing mandoline, slice into about ¹⁄₁₆”-thick rounds. Pour a 3rd of the cream combination right into a 13×9″ baking dish. Prepare potato slices in dish, fanning out a handful at a time so that they overlap barely and urgent down gently in between additions to compress, till all potatoes have been used and backside of dish is roofed. Pour remaining cream combination over; press down on potatoes so that they’re principally submerged. Cowl dish tightly with foil; bake till potatoes are tender, 60–70 minutes (a tester inserted within the center ought to meet with barely any resistance). Take away foil, scatter 3 oz. sharp yellow cheddar, coarsely grated (about 1 cup), over, and proceed to bake, uncovered, till cheese is melted, 6–8 minutes extra.
Step 4
Let scalloped potatoes cool 5 minutes, then prime with small dollops of ⅓ cup bitter cream and scatter 4 scallions, darkish inexperienced elements solely, thinly sliced on a diagonal, and bacon over.
Do Forward: Scalloped potatoes (with out cheese and toppings) could be made 1 day forward. Let cool; cowl and chill. Reheat, coated, in a 350° oven 20 minutes, then uncover, prime with cheese, and cook dinner till melted, 7–9 minutes.
