Preheat oven to 350 degrees F.

With a sharp knife, slice off the root and tip ends of the white onion and peel. Carefully slice the onion halfway, root to tip, so that individual layers of the onion can be separated. Take off the four outermost layers and set them aside to become "bombshells." Finely chop the remaining center of the onion.

In a large bowl combine ground beef, egg, chopped onion, chopped mushroom, minced garlic, Smashburger Seasoning, and parmesan cheese. Mix by hand until just blended, don't overwork. Form into four balls, sizing each so that it fits into one of the bombshells so the onion shell still closes around it. 

Assemble bombs by inserting the meatball into the onion shell. Wrap vertically with a strip of bacon so the bacon covers the "seam" in the onion. Wrap with 1-2 more pieces of bacon, depending on the size of the onion bomb. Use 1-2 wooden toothpicks to secure the outermost bacon strip. Place on a lightly-greased, rimmed cookie sheet. Bake 40 minutes. 

Insert a temperature probe, and continue baking until the internal temperature of the largest bomb reaches 160 degrees. Remove the baking sheet from the oven and sprinkle onion bombs with shredded cheese. Return to the oven for another five minutes to melt the cheese. (If desired, broil to make the cheese brown and bubbly.) Remove and allow the onion bombs to rest 10 minutes before serving.

To serve: 

Lots of choices here. Drizzle with your favorite barbeque sauce, or serve on a bun as a super round "burger." Or interpret this as a giant meatball and serve with marinara sauce and pasta. Or think meatloaf and park it next to mashed potatoes and gravy. The Smashburger Seasoning is delicious enough to stand alone, without adding other spice profiles, so consider keeping it simple!

Alternative Preparation:

Consider smoking these on a grill at about 300 degrees. This will take a lot longer, but have delicious results, especially if glazed with your favorite barbeque sauce toward the end of the process.