12 large eggs
1/2 cup half-and-half
1 teaspoon kosher or sea salt
1/2 teaspoon freshly ground pepper
3 tablespoons extra-virgin olive oil
1 small yellow onion, halved lengthwise and cut into thin wedges
6 ounces baby spinach leaves, coarsely chopped (about 4 cups firmly packed)
1 pint cherry tomatoes, stemmed and halved
1 1/2 tablespoons chopped fresh oregano
1/2 cup (about 3 ounces) crumbled feta cheese
In a large bowl, whisk together the eggs and half-and-half until thoroughly combined. Whisk in the salt and pepper. Set aside.
Position an oven rack about 3 inches below the heat source and preheat the broiler.
In a 12-inch ovenproof skillet, preferably nonstick, heat the olive oil over medium heat. Swirl to coat the pan, add the onion, and sauté, stirring occasionally, until the onion softens, about 2 minutes. Add the spinach a handful at a time, and sauté just until wilted and bright green, about 3 minutes. As soon as the spinach is wilted, add the tomatoes and oregano and sauté, stirring constantly, until the tomatoes are heated through, about 1 minute.
Add the egg mixture to the pan, reduce the heat to medium-low, and cook, without stirring, until the frittata is set on the bottom and around the edges, about 4 minutes. While the frittata is cooking, simultaneously lift one edge of the frittata with a spatula and tilt the pan a little so the uncooked egg flows under the set edge. Repeat this at regular intervals around the edge of the frittata.
When the eggs are mostly cooked, scatter the feta cheese over the top and place the frittata under the broiler. Broil until the top is golden brown and the eggs are set but still moist, about 3 minutes.
Allow the frittata to rest for 5 minutes, then slice it into wedges and serve immediately on warmed plates.