Reduce oven temp to 325 degrees.
Cook bacon until crisp in a large skillet over medium-high heat, about 6 minutes. Transfer to a paper towel-lined plate.
Remove all but 1 tablespoon bacon fat from the pan and add the shallot. Cook, stirring often until the shallot is lightly browned, about 2 minutes
Add the spinach and garlic and, stirring often, cook until the spinach has wilted and most of the liquid has cooked off, about 3 minutes. Remove from the heat and season with a pinch of salt and pepper.
Add the remaining three eggs to the large bowl with the egg yolk. Thoroughly whisk until uniform.
Add the half and half, dijon, and salt and pepper and gently whisk to combine.
Place the pie pan onto a baking sheet. Evenly sprinkle the bacon, spinach and cheese into the pie crust. Fill with the egg and half and half mixture.
Bake on the bottom rack of the oven until the edges of the quiche are just set and the center has a slight jiggle or using an instant-read thermometer, cook until the center of the quiche reaches 170 degrees, about 1 hour.
Allow the quiche to cool for 1 hour before serving.