Heat oven to 375° and line a baking sheet with a non stick baking mat or parchment paper. Heat a skillet on medium high and add the butter. Once hot, fry the minced onion until it begins to brown, then remove from heat and set aside.
Combine almond flour, baking powder, baking soda, onion powder, onion flakes and sea salt in a medium sized microwave safe bowl and mix well. Toss in shredded mozzarella, then place the cream cheese on top. Microwave for 1 minute. Mix with a wooden spoon, then microwave for an additional 30 seconds.
Mix again, and if the mozzarella is not fully melted, microwave in 15 second intervals until it is. Fold in 1 egg and 1 egg white and once fully combined, mix in the vinegar just until incorporated. *see notes
Divide into four and form into buns, see photos and video above for correct shape. They will spread while baking so be sure not to form them too large. Place evenly on a baking sheet with a non stick baking mat, then brush the tops with the cooked onion/butter. Bake for 14-16 minutes, until the tops of the buns have browned.