How to Make It
Heat oven to 400° F. Heat 2 tablespoons of the oil in a large ovenproof nonstick skillet over medium heat. Add the potatoes and onion and cook, tossing occasionally, until the potatoes are tender, 12 to 15 minutes.
Meanwhile, in a large bowl, whisk together the eggs, ½ teaspoon salt, and ¼ teaspoon pepper. Mix in the spinach, Cheddar, and ham.
Add the egg mixture to the skillet, stir once, and transfer the skillet to oven. Cook until the eggs are set, 12 to 14 minutes.
Meanwhile, in a large bowl, whisk together the vinegar, mustard, the remaining tablespoon of oil, and ¼ teaspoon each salt and pepper. Add the lettuce and toss to coat. Serve with the frittata.