Step 1: Preheat your oven to 375°F (190°C). If using an unbaked pie crust, prick the bottom with a fork several times. If using a pre-baked crust, you can skip this step.
Step 2: In a small skillet, heat olive oil over medium heat. Add chopped onion and sauté until softened, about 3-5 minutes. Remove from heat.
Step 3: Sprinkle half of the Monterey Jack cheese and half of the cheddar cheese into the bottom of the pie crust. Top with the sautéed onions and diced green chiles. Sprinkle the remaining cheeses over the chiles and onions.
Step 4: In a medium bowl, whisk together the eggs, heavy cream (or half-and-half), salt, black pepper, and optional cayenne pepper until well combined.
Step 5: Carefully pour the egg mixture over the ingredients in the pie crust. Place the quiche on a baking sheet to catch any potential spills.
Step 6: Bake for 45-55 minutes, or until the center is set and a knife inserted near the center comes out clean. The top should be golden brown.
Step 7: Remove from the oven and let the quiche rest for 10-15 minutes before slicing and serving. This allows the quiche to set properly.