Bring 5 cups of water to a boil in a small pot, seasoning with 2 tsp salt. This is separate from the salt in the egg mixture.

Heat olive oil in an 11-inch cast iron skillet over medium heat. Add chopped onions and cook until soft and translucent, about 5 to 8 minutes. Turn the burner off if they finish early.

Once the water reaches a rolling boil, add broccoli florets. Cover immediately and set a timer for exactly 3 minutes. Do not lift the lid. Drain right away into a colander and do not rinse.

Whisk together eggs, milk, 0.75 tsp salt, cayenne, and black pepper in a mixing bowl until fully combined.

Preheat your broiler and move the oven rack to about 5 inches from the flame.

Layer the blanched broccoli over the cooked onions. Add the chopped ham on top, then scatter the cheese chunks evenly across the surface.

Turn the burner back to medium. Make sure the pan is hot before pouring in the egg mixture. A hot pan is key to getting the edges to set properly.

Pour the egg mixture over everything. Do not stir. Cook undisturbed for 7 to 9 minutes until the edges are visibly set and the center still looks slightly wet.

Move the skillet to the broiler. Broil for 2 to 5 minutes, checking every minute. The frittata is done when the top is golden brown and the center is fully set with no jiggle. Let it rest 2 to 3 minutes before slicing.