Step 1

Adjust oven rack 6 inches from broiler element and heat broiler. Microwave spinach and ½ cup water in large covered bowl until spinach is wilted and decreased in volume by half, about 4 minutes. Remove bowl from microwave and keep covered for 1 minute. Carefully remove cover, allowing steam to escape away from you, and transfer spinach to colander set in sink. Using back of rubber spatula, gently press spinach against colander to release excess liquid. Transfer spinach to cutting board and coarsely chop. Return spinach to colander and press a second time.

Step 2

Beat eggs, half-and-half, ¾ teaspoon salt, and ½ teaspoon pepper together with fork in bowl until thoroughly combined and mixture is pure yellow; do not overbeat. Stir in ½ cup feta.

Step 3

Heat 10-inch cast-iron skillet over medium heat for 3 minutes. Add oil and heat until shimmering. Add onion and cook until softened, about 5 minutes. Stir in garlic and oregano and cook until fragrant, about 30 seconds. Stir in spinach and cook until uniformly wilted and glossy, about 2 minutes.

Step 4

Add egg mixture and, using heat-resistant rubber spatula, constantly and firmly scrape along bottom and sides of skillet until eggs form large curds and spatula leaves trail on bottom of skillet but eggs are still moist, 1 to 2 minutes. Shake skillet to distribute eggs evenly. Sprinkle with remaining ¼ cup feta and gently press into eggs with spatula.

Step 5

Transfer skillet to oven and broil until center of frittata is puffed and surface is just beginning to brown, 3 to 5 minutes; when cut into with paring knife, eggs should be slightly wet and runny.

Step 6

Using potholders, transfer skillet to wire rack and let frittata rest for 5 minutes. Being careful of hot skillet handle, run spatula around edge of skillet to loosen frittata, then slide frittata out of skillet onto cutting board. Cut frittata into wedges and serve warm or at room temperature.