Preheat your oven to 350°F. While it heats, prepare your mise en place: pat the roasted red peppers dry with paper towels to remove excess moisture (this prevents a watery quiche), slice the scallions into 1/4-inch rings keeping the white and green parts separate, shred the cheddar cheese if not already done, and finely chop the fresh parsley. Bring the eggs to room temperature by letting them sit out for 10 minutes—room temperature eggs blend more smoothly into the custard, creating a silkier texture.

Place the thawed pie crust into a pie plate. Bake it in the preheated oven for 6-8 minutes until it begins to set but is not yet golden—this prevents a soggy bottom crust when the wet custard is added. Remove from the oven and lightly dust the entire interior surface with the all-purpose flour, which acts as a moisture barrier. Return to the oven for just 1-2 minutes to lightly set the flour, then set aside to cool slightly.

Spread the dried roasted red peppers evenly across the bottom of the partially baked crust. Scatter the cooked sausage crumbles over the peppers, followed by the shredded cheddar cheese. Sprinkle the white parts of the scallions (the milder, more delicate portions) over the cheese layer, reserving the green parts for the egg mixture. This layering approach distributes flavors evenly throughout the quiche and ensures the sausage and peppers stay nestled in the filling rather than floating to the top.

In a medium bowl, whisk together the room-temperature eggs, half and half, kosher salt, freshly ground black pepper, ground mustard, and the reserved green parts of the scallions along with the fresh parsley. Whisk until the mixture is smooth and well combined, about 1-2 minutes—I prefer using a whisk rather than a blender for quiches because it incorporates air gently, creating a lighter, more delicate custard. Avoid over-mixing, which can make the texture tough.

Pour the custard mixture from Step 4 evenly over the filling ingredients from Step 3, filling the crust until just below the rim. Place the quiche in the preheated 350°F oven and bake for 50-55 minutes. The quiche is done when the center is just set with a slight jiggle in the very middle when you gently shake the pan—the residual heat will continue cooking it as it cools, so don't overbake or it will become rubbery. The top should be lightly golden.

Remove the quiche from the oven and let it cool in the pie plate at room temperature for at least 30 minutes before slicing. This resting period allows the custard to fully set and makes clean, neat slices possible. The quiche can be served warm or at room temperature, and it will hold its structure better after cooling.