Step 1: Prepare the Onions. Slice off the top 1/4 inch from each onion and peel the outer layer. Carefully hollow out the center of each onion, leaving a 1/2 to 3/4 inch thick shell. Chop the scooped-out onion flesh and set aside about 1/2 cup for the beef mixture.
Step 2: Preheat Oven/Grill. Preheat your oven to 375°F (190°C) or prepare your grill for indirect medium heat (if grilling, wrap each bomb in foil).
Step 3: Cook the Beef. In a large bowl, combine ground beef, 1/2 cup chopped onion, garlic powder, onion powder, smoked paprika, salt, and pepper. Form into 4 equal patties. Heat 1 tbsp olive oil in a skillet over medium-high heat and sear each beef patty for 2-3 minutes per side, just to brown. They will finish cooking inside the onions.
Step 4: Assemble the Onion Bombs. Drizzle the inside of each hollowed onion shell with olive oil and season with a pinch of salt and pepper. Place one seared beef patty into the bottom of each onion. Top with 1/4 cup shredded cheddar cheese, followed by half of the prepared mac and cheese (approx. 1/2 cup). Sprinkle with crumbled bacon. Repeat with another layer of mac and cheese and a final sprinkle of cheddar cheese.
Step 5: Bake or Grill. Place the stuffed onions in a baking dish or on a grill-safe tray. Bake for 50-60 minutes, or until the onions are very tender and the internal temperature of the beef reaches 160°F (71°C). If grilling, cook for 45-60 minutes over indirect heat.
Step 6: Serve. Remove from oven or grill and let rest for a few minutes before serving hot. Enjoy your Loaded Bacon Mac Cheeseburger Stuffed Onion Bombs!