Preheat oven to 375°F.
(Optional but recommended) Blind bake the crust for 8–10 minutes for a crisp bottom.
Heat a small skillet over medium heat with a touch of oil. Sauté the diced onion 3–4 minutes, just until softened (not browned). Let cool slightly.
Evenly scatter ham, sautéed onion, and cheese in the crust.
In a bowl, whisk together eggs, cream, salt, pepper, and any optional seasonings.
Pour the egg mixture over the fillings.
Bake for 45–55 minutes, until the edges are set and the center is just barely jiggly.
Let rest 10 minutes before slicing.