Position a rack in the middle of the oven; preheat to 375 degrees .
In a 10-to 12-inch ovenproof skillet, add the EVOO, three turns of the pan, and swirl to coat the entire surface, using a paper towel to reach the edges. Heat over medium. Add the onion, garlic and thyme; season with salt and pepper. Cook, stirring often, until onion softens, 6 to 7 minutes. Add the spinach and stir to wilt, 1 to 2 minutes.
In a large bowl, whisk the eggs, ricotta, Romano and nutmeg until smooth; season. Pour the egg mixture over the spinach.
Bake until the center is just set, 15 to 20 minutes. When cool, run a knife around the skillet edges to release the frittata. Invert onto a work surface and cut into wedges.