Potato and Onion Frittata

Heat oven to 400 degrees F. Thinly slice 1/4 of the onion and set aside. Chop the remaining 3/4 onion. Heat 1 tablespoon oil in a large oven-safe nonstick skillet (we used a 12-in. cast iron) over medium heat. Add the potatoes and chopped onion, season with 1/4 teaspoon each salt and pepper and cook, covered, stirring occasionally, for 7 minutes. Uncover and continue cooking, stirring occasionally, until the potatoes are golden brown and just tender, 12 to 15 minutes more. Stir in the parsley. While the potatoes and onions are cooking, in a medium bowl, whisk together the eggs, milk, and 1/4 teaspoon each salt and pepper; stir in the cheese. Pour the egg mixture into the skillet and stir to distribute the ingredients. Transfer the skillet to the oven and bake until the frittata is puffed, brown around the edges and a knife inserted into the center comes out clean, 14 to 16 minutes. Meanwhile, in a large bowl, whisk together the vinegar, mustard, the remaining tablespoon oil and 1/4 teaspoon each salt and pepper. Add the sliced onion and toss to coat. Just before serving, add the cucumber and lettuce to the onions and toss to combine. Serve with the frittata.