Look for a long-necked squash for this Spanish-style egg tortilla—it’s so much easier to slice thinly (save the bottom for another use). Slideshow: Butternut Squash Recipes
Preheat the oven to 400°.
Heat 2 tablespoons of the olive oil in a large ovenproof nonstick skillet over moderate heat, swirling the oil to coat the skillet.
Arrange some slices of butternut squash in one layer over the bottom of the skillet. Top with some of the chorizo and season with salt and pepper. Continue layering in this manner until all the squash and chorizo have been used. Drizzle the remaining 1 tablespoon of olive oil over the top layer of squash. Reduce the heat to moderately low, cover and cook undisturbed until squash is tender when pierced with the tip of a knife, about 15 minutes.
While the squash is cooking, whisk together the eggs and milk in a small bowl. Season lightly with salt and pepper.
Pour the egg mixture evenly over the squash, tilting the skillet to help it slip between the squash layers. Cook until the eggs begin to set, then put the skillet in the oven and cook until the egg mixture is fully set, about 10 minutes.
Remove from the oven and let rest about 10 minutes before inverting the tortilla onto a serving platter. Serve warm or at room temperature.