Preheat oven to 325 degrees F (165 degrees C). Line a baking sheet with parchment paper.

Melt butter in a pan over medium-high heat. Saute diced onion, celery, mushrooms, and 1 teaspoon salt until the mixture dries out and starts to turn golden brown, 5 to 7 minutes. Stir in garlic, black pepper, thyme, and cayenne pepper. Remove from heat and let cool to room temperature, about 10 minutes.

Combine remaining 2 teaspoons salt, ground beef, bread crumbs, egg, buttermilk, 2 tablespoons ketchup, and Worcestershire sauce in a large bowl. Add cooled vegetable mixture. Mix with your hands until thoroughly combined. Cover with plastic wrap and place in the refrigerator.

Slice 2 round sides, about 1/2-inch-thick, off the second onion for the zombie eyes. Push 1 clove into the center of each slice. Cut a 1-inch-wide section off the remaining onion and dice into chunks to form zombie teeth.

Transfer meatloaf mixture to the prepared baking sheet and use damp hands to shape into a zombie skull shape that is 3 to 4 inches tall. Form indents to represent the eyes, nose, and mouth. Drape bacon over the face, covering most of the meatloaf. Add onion eyes and teeth. Tuck in loose ends of bacon under the sides with a spatula. Cover the onion eyes with aluminum foil to prevent browning.

Bake in the preheated oven until the meatloaf reaches an internal temperature of 155 degrees F (68 degrees C), about 1 hour.

Stir barbeque sauce, 1/4 cup ketchup, and hot sauce together in a bowl to make the blood sauce.