Cook bacon in a small nonstick skillet over medium heat, stirring occasionally, until most of fat is rendered and bacon begins to brown, about 5 minutes. Set 1 tablespoon of dark-green part of scallions aside. Add remaining scallions to skillet and cook, stirring occasionally, until bacon is crisp and scallions are soft, 4-5 minutes longer. Using a slotted spoon, transfer bacon mixture to a paper towel-lined plate.
If you have bacon grease left add to the olive oil and measure 3 tablespoons Add butter. Place 2 tablespoons fat mixture in skillet. Pour 2 tablespoons into another bowl; add bacon mixture. Reserve remaining 2 tablespoons in small bowl.
Coarsely grate potatoes into a large bowl. Working in batches, squeeze potatoes with your hands to release liquid; transfer to another large bowl. Add the reserved bacon mixture, 1 teaspoon salt, and pepper; toss to evenly distribute.
Heat reserved skillet over medium-low heat. Scrape potato mixture into hot skillet, pressing with a spatula into an even layer. Tent with foil; cook for 10 minutes, rotating skillet frequently to avoid hot spots. Remove foil; cook rösti until bottom is golden brown, about 5 minutes longer. Run a silicone spatula around sides and underneath rösti. Invert a large plate over skillet. Using oven mitts, carefully flip rösti onto plate.
Heat remaining 2 tablespoons fat in skillet. Slide rösti back in, browned side up. Tent with foil; cook for 5 minutes. Uncover; cook until browned on bottom and potatoes are cooked through, about 10 minutes longer. Gently run spatula around sides and underneath rösti and slide onto a serving plate. Sprinkle with reserved scallions and season with more salt, if desired.
