Preheat your oven to 400°F (200°C).
Toss the halved Brussels sprouts in olive oil, salt, and pepper, then spread them on a baking sheet.
Roast the Brussels sprouts for about 20-25 minutes, until they are golden brown and tender.
While the sprouts roast, cook the chopped bacon in a skillet over medium heat until crispy, then drain on paper towels.
In a large mixing bowl, whisk together the eggs and heavy cream until fully combined.
Stir in the roasted Brussels sprouts, crispy bacon, and half of the Gruyère cheese to the egg mixture.
Pour the filling into the pre-made pie crust, then sprinkle the remaining Gruyère cheese on top.
Bake for 30-35 minutes, or until the custard is set and the top is lightly golden.
Let the quiche cool for at least 10 minutes before slicing and serving.