Heat 1 1/2 tablespoons vegetable oil in a skillet over medium heat. Add the spinach and cook until wilted, 1 to 2 minutes. Transfer to a colander set in a large bowl; refrigerate until cold, about 15 minutes. Squeeze out any excess moisture; set the spinach aside.
Meanwhile, heat the remaining 1 1/2 tablespoons vegetable oil in a skillet over medium-low heat. Add the leeks, sprinkle with 1/2 teaspoon salt and cook until soft, about 10 minutes. Add the scallions and cook 3 to 4 more minutes. Let cool.
Peel the potatoes and grate on the large holes of a box grater. Transfer to a clean kitchen towel, gather into a pouch and twist closed, then squeeze over the sink to remove as much liquid as possible. Transfer to a large bowl; add the leek-scallion mixture, spinach, eggs, flour, dill, feta, 1 teaspoon salt and 1/4 teaspoon pepper and mix with your hands until combined.
Heat 1/2 inch vegetable oil in a large skillet over medium-high heat until a deep-fry thermometer registers 350 degrees F. Working in batches, scoop about 2 tablespoons of the potato mixture into the pan for each latke and flatten with a spatula; fry until golden brown and cooked through, 1 to 2 minutes per side. Drain on paper towels. (Return the oil to 350 degrees F between batches.) Serve with tzatziki.
Photograph by Con Poulos
Recipe adapted from Estia for Food Network Magazine