Grate potatoes and onion on the large holes of a box grater into a large bowl. Squeeze out excess liquid from potatoes and onions with a dish towel into a medium bowl; reserve liquid. Add egg yolks, garlic, Parmesan, and lemon zest to potatoes and onion and mix well; season with salt. Gently fold in flour, adding reserved liquid if needed, until mixture is loose but still holds together.
Heat ¼ cup oil in a medium nonstick skillet over medium. Squeeze about ⅓ cup potato mixture into a rough ball with your hands to get rid of excess liquid. Press ball down to make a 3½”-diameter flat round. Working in batches, cook latkes, turning occasionally and adding 1–2 Tbsp. oil between batches, until both sides are golden and crunchy, about 4 minutes per side. Transfer to paper towels to drain.
Process ricotta in a food processor until super smooth and silky. Serve alongside latkes.