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 over medium high heat. Don't let it burn! If it begins to sputter excessively, turn heat to low. Once butter begins to turn brown and smell nutty, swirl in cumin and then vinegar. Let cook one more minute and then remove from heat.
Place avocado on small plate flesh side up. Pour brown butter into each pit cavity evenly and then grate some queso fresco over. Eat with a spoon!