Carefully slice the avocado evenly in half and remove pit.
Heat a cast iron pan until hot, swirl in grapeseed oil, and place avocado, meat side down in pan. Turn to low and let sit while you toast butter.
In a small pan, heat butter on medium high until foaming. Be careful not to burn butter, turn to low if bubbles start sputtering excessively. Just when butter begins to turn brown, swirl in cumin and then vinegar. Let cook one more minute, then remove from heat.
Remove avocado from cast iron pan and place on plate. Pour browned butter into pitt cavity of each side evenly...Grate a little queso fresco on top, and eat with a spoon.