Heat oven to 375°F and set a rack in the lower-middle position.
Roll the pie dough, fit it into a 9-inch pie plate, and crimp the edges. Chill 10 minutes.
Line crust with parchment and fill with pie weights. Blind-bake 15 minutes, then remove weights.
Dock the crust with a fork and bake 8–10 minutes more until lightly golden. Cool slightly. Brush Dijon inside if using.
Cook bacon in a skillet over medium heat until crisp, 6–8 minutes. Transfer to paper towels; reserve 1 tbsp drippings.
Soften onion in the bacon drippings over medium heat, 3–4 minutes. Let cool.
Whisk eggs, cream, and milk. Season with salt, pepper, and nutmeg until smooth and well combined.
Reduce oven to 350°F. Scatter bacon, onion, and two-thirds of the cheese in the crust. Pour in custard; top with remaining cheese.
Bake 30–35 minutes until set at the edges and slightly jiggly in the center. Rest 15 minutes. Garnish with chives, slice, and serve.
Notes
Variation: Swap half the Gruyère for sharp Swiss or add a pinch of fresh thyme. For a smoky twist, use applewood bacon. Storage: Refrigerate leftovers up to 3 days; reheat at 300°F until warmed through or enjoy at room temperature.This recipe is an original creation inspired by classic Easy Quiche Lorraine Recipe flavors. All ingredient ratios and instructions are independently developed.