Preheat the oven to 425ºF (220ºC).
Add the bacon to a 10-inch skillet over medium-high heat (no need to preheat the pan). Cook until crispy. Use a slotted spoon to transfer the bacon to a paper-towel lined plate. Spoon off all but 1 tablespoon of the bacon grease (see note).
Add the onions and butter to the pan with the remaining bacon grease, and reduce the heat to medium-low. Season with salt and pepper. Cook, stirring occasionally, until the onions are softened and deeply golden, 8 to 10 minutes. Set the pan aside, and let the onions cool for a few minutes.
Whisk the egg and cream together in a medium bowl. Season with salt and pepper. Stir in the onions, bacon, and thyme.
Roll out one of the pie crusts into a 9-inch circle. Fit it into an 8-inch pie plate.
Spread the filling into the dish.
Roll out the remaining crust into an 8-inch circle. Cut into 10 strips. Lay 5 strips an even length apart from each other over the filling. Lay the remaining strips on top in the opposite direction (perpendicular to the first set up strips).
Trim the edges of the crust, and flute, or crimp closed as desired. Brush the top of the pie with the egg wash mixture.
Bake for 15 minutes, then reduce oven temperature to 350ºF (180ºC). Bake for 23 to 25 minutes, until the crust is golden, and filling bubbly. Let cool for at least 5 minutes before serving, but also lovely eaten at room temperature.
Use It Up: Bacon Grease 101
Like any other oil, leftover bacon grease can be used to sauté, but it has the added benefit of infusing a smoky, porky flavor. It seriously ups your popcorn game if you use it to pop the kernels on the stovetop. It’s also the secret ingredient to my chili. I keep a jar in the fridge, and add to it whenever I make bacon.