Dice 1/2 white onion and mince 3-4 garlic cloves. Set aside.
In a small bowl, combine kosher salt, pepper, onion powder, paprika, and chili powder to create the rub.
Heat a griddle or skillet over medium heat. Add avocado oil and diced onions. Season with a pinch of salt and cook for 4-6 minutes until soft and translucent.
Add ground beef to the skillet with onions. Cook for 3-4 minutes, breaking up the beef. Sprinkle with the rub and mix well. Continue to cook for another 2-3 minutes.
Add tomato sauce and tomato paste to the skillet. Stir to combine and cook for 2 minutes.
Add ketchup, mustard, Worcestershire sauce, and BBQ sauce. Mix thoroughly and cook until the mixture thickens to your liking, about 5-7 minutes. Remove from skillet and place in a bowl to keep warm.
Lay out brioche sandwich slices. Place a slice of mozzarella cheese and a slice of gouda cheese on each slice. Spoon a generous amount of sloppy joe mixture onto the cheese. Top with another slice of gouda cheese, another slice of mozzarella cheese, and the top slice of bread.
Clean the skillet and return it to medium heat. Add butter and melt. Place sandwiches in the skillet, pressing down lightly. Toast one side until golden brown, about 2 minutes. Flip and toast the other side until golden brown and cheese is melted, about 2 more minutes. Remove from skillet.