Heat oven to 355 F / 180 C.
Heat 1 tablespoon of the oil in a medium ovenproof nonstick skillet over medium-high heat.
Add the onion and a pinch of salt and pepper, cook while stirring occasionally, until tender, 3 to 4 minutes.
Add the spinach, cook while tossing, until wilted, 1 to 2 minutes.
Add the beaten eggs and sprinkle with goat’s cheese.
Cook until the mixture starts to set around the edges, around 1 to 2 minutes.
Transfer to the oven and bake until set, 10 to 12 minutes.
Mix together the vinegar, 2 tablespoons of oil, and a pinch of salt and pepper in a large bowl.
Add the greens and toss to cover the greens with the mixture.