Heat the oven to 375 degrees F.
Pour 3 tablespoons olive oil into a 12-inch non stick, ovenproof skillet over medium-high heat. Add the chorizo and fry for 5 minutes; add the potato and onion and fry until the onions start to soften, about 5 minutes. Reduce the heat and cover the skillet; cook until the potatoes are tender, about 10 minutes. Remove the cover, raise the heat, and cook until the potatoes are brown and crispy, about another 5 minutes. Meanwhile break the eggs into a large bowl, add the milk, season with salt and pepper, and whisk until frothy. Pour the egg mixture into the pan, shaking the pan to distribute the eggs evenly. Put the pan into the oven and cook until puffy and set, about 12 to 15 minutes. Turn out onto a cutting board and let it cool to room temperature. Toss the baby greens with the remaining oil and lemon juice; season with salt and pepper. Place the greens on top of the tortilla, cut it into thin wedges, and serve.