Step 1

Arrange a rack in the middle of the oven and heat to 400°F.

Step 2

Place the eggs, cream, dill, salt, and pepper in a medium bowl and whisk to combine; set aside.

Step 3

Heat the oil in an 8-inch cast iron or oven-safe nonstick skillet over medium until shimmering. Add the onion and cook, stirring occasionally, until softened, about 5 minutes. Add the spinach (in batches if needed), and toss until wilted and almost all of the liquid is evaporated, about 3 minutes for fresh spinach. Spread into an even layer over the bottom of the skillet and sprinkle with the cheese.

Step 4

Pour the egg mixture over the vegetables and cheese. Tilt the pan to make sure the eggs settle evenly over all the vegetables. Cook, undisturbed, until the eggs at the edges of the pan begin to set, about 2 minutes.

Step 5

Transfer the pan to the oven and bake until the eggs are completely set, 8 to 10 minutes. To check, cut a small slit in the center of the frittata. If raw eggs run into the cut, bake for another few minutes; if the eggs are set, pull the frittata from the oven. Cool in the pan for 5 minutes, then slice into wedges and serve warm.