Place the ground beef in a large bowl and season with the salt, pepper and garlic powder. Mix gently to coat and then divide the beef into four equal sections. Bread apart each section and form it into two patties – one for the top and one for the bottom. Place a round of goat cheese in the center of one patty and cover it with the other patty, pressing gently around the edges to seal the goat cheese in. Repeat with remaining beef.
At this point you can grill the burgers – I chose to use the skillet that the onions were cooked in. Heat a skillet over medium-high heat and add the olive oil and butter. Cook the burgers until your desired doneness (we like medium well, at least 4 minutes per side – but remember the goat cheese is inside!), and with 1 minute before they are finished, place a few cheddar slices on top.
To assemble the burgers, spread a few tablespoons of guac on the bottom bun and place the burger on top. Cover with some caramelized onions, BBQ mayo and spring greens. Serve immediately!