Chop ham into ½-inch pieces or smaller.

Peel onion, cut in half, and slice into thin half-moons.

Grate gruyere cheese.

Heat a 10-inch nonstick, oven-safe skillet over medium-low, then add butter.

When butter melts, add the sliced onions and sauté. Cook until onions are lightly caramelized, about 15 minutes.

Meanwhile, whisk the eggs, cheese, dijon mustard, and cream together in a medium bowl, with ¼ teaspoon pepper.

Add the chopped ham to the egg and cheese mixture and stir, then pour into the pan with the onions. Spread the filling around so it’s evenly distributed through the eggs.

Preheat the broiler and set the oven rack a few inches underneath the heating element.

Turn up the heat under your skillet to medium high.

Cook the frittata on the stove for 5-8 minutes. The best way to do this is to slowly lift the edges of the frittata with your spatula, tip the pan, and let the uncooked egg mixture pool underneath. Do that a few times in different spots, as it cooks. Turn down the heat if it looks like your frittata is browning too quickly on the bottom.

When the bottom is totally set, and the egg mixture on top is a little more than halfway cooked, you’re ready to put it under the broiler. Note: it’s ok for the top to still look raw.

Sprinkle on the parmesan cheese.

Slide your pan onto the oven rack, and broil. Watch closely because this can happen fast: take it out when the frittata puffs up pleasingly, looks entirely cooked, and has lightly browned on top, 1-3 minutes.

Take it out, and let it rest for a few minutes. It will continue to cook in the pan as it sits.

Give it a quick hard shake and slide it out onto a cutting board. Slice and eat!