Preheat oven to 425 degrees. In a 10-inch ovenproof nonstick skillet, heat oil over medium-high. Add leek and potato, season with salt and pepper, and cook until leek is translucent, about 5 minutes.
Add eggs and ricotta, season with salt and pepper, and stir to combine. Cook, undisturbed, until edges are set, about 2 minutes. Transfer skillet to oven and bake until top of frittata is just set, 10 to 13 minutes. Invert or slide frittata onto a plate and cut into 6 wedges. Serve warm or at room temperature.
The frittata will appear to be mostly liquid on top when you transfer it to the oven, but as long as the edges are set, the eggs will finish cooking evenly.