Preheat oven to 400°F (200°C). Line a 9-inch tart pan with pastry, prick the base with a fork, line with parchment and pie weights, and blind bake for 15 minutes. Remove weights and bake 5 more minutes until lightly golden.

Melt butter in a skillet over medium-high heat. Add onion, garlic, and bacon. Cook until bacon is light golden and onion is softened. Transfer to a paper towel-lined bowl to cool.

Whisk together the eggs, heavy cream, salt, and pepper until well combined.

Scatter the cooled bacon and onion mixture across the base of the baked pastry shell. Sprinkle the Gruyère cheese evenly over the top.

Carefully pour the egg mixture over the filling, gently pushing some cheese and bacon below the surface.

Bake for 35 to 40 minutes until the quiche is golden around the edges and the center has a gentle wobble. Rest 10 minutes before slicing and serving.