How to Make It
Preheat the oven to 400°F. Heat 1 tablespoon of the oil in a nonstick ovenproof skillet over medium-high. Add the asparagus and cook, stirring occasionally, until softened, about 3 minutes. Add the broccoli and tomatoes; cover and cook, stirring occasionally, until the tomatoes burst and the broccoli softens, 5 minutes more.
Whisk together the eggs, 1 teaspoon of the salt, and ½ teaspoon of the pepper. Pour over the vegetables in the skillet and cook, stirring gently, until the eggs just begin to set, about 1 minute. Sprinkle with the goat cheese and transfer the pan to the oven. Bake until the center is set, 10 to 12 minutes.
Meanwhile, whisk together the vinegar and the remaining salt, pepper, and oil in a large bowl. Toss with the watercress to coat. Cut the frittata into wedges. Serve with the watercress and bread.