These flavorful burgers are classy but not too classy. :) Grill them or cook in a blazing-hot cast-iron pan. You can make the bacon and onions ahead of time if you like and reheat for a few seconds in the microwave or in a pan on the grill right before serving. (Recipe contains affiliate links.)
Preheat oven to 375°F. Line a half sheet pan with foil. To make the bacon weave, start by laying four bacon slices next to each other, long sides touching, in the middle of the pan. Then, one by one starting at the top, lay each of the remaining pieces crosswise. Weave the first piece over, under, over, under the original slices. Weave the second piece under, over, under, over, etc. (If you learned a different weaving method in elementary school, go for it.) Bake bacon until done to your liking -- 15 minutes plus depending on the thickness of the cut. Drain on paper towels, reserving the grease in the pan. When cooled enough, cut weave into four squares.
Meanwhile, form four burger patties from the ground beef. Use your thumb to create a nice big divot in the middle of each patty to ensure even cooking. Sprinkle generously with salt and pepper immediately before cooking. You can grill the burgers or cook in a cast-iron pan with a couple tablespoons of the bacon grease. To grill, prepare a hot side and a cooler side of the grill with a little oil on the grates. Start burgers over direct heat, flipping once, to form a nice crust on both sides. This should take about three minutes per side. Transfer burgers to indirect heat to cook through to your liking (125°F for medium-rare, 130°F for medium on an instant-read thermometer). For cast iron, preheat a 12-inch cast iron pan over high heat until very hot. Pour in two tablespoons of bacon grease, then set burgers into pan. Cook on high heat to form a crust on both sides, flipping once, then reduce heat to medium and continue cooking according to internal temperature instructions above. Let burgers rest for at least 5 minutes before continuing.
While burgers rest, add another two tablespoons of bacon grease into the hot pan, pile in the onions and sprinkle generously with salt and pepper. Cook, stirring frequently, until browned and wilted. Add marsala and cook until evaporated.
To assemble the burgers, toast the buns for a minute or two if you like, then layer on some spinach leaves, a burger patty, bacon, onions, and some crumbled blue cheese.
Techniques adapted from Kenji Lopez-Alt at Serious Eats.