In a bowl combine grass fed ground beef, garlic powder, onion powder, salt and pepper to taste, being careful not to over mix. Adjust seasoning, if needed, then form 4 even thin patties, about 1/2-inch thick, and set aside.
Preheat skillet to medium-high heat.
Slice onions and mushrooms, if not pre-sliced.
Place 2 tablespoons of butter into the skillet. Sauté both the onions and mushrooms and cook until tender and caramelized (approximately 3-5 min).
Grill burgers over medium-high heat, flipping once until desired doneness is reached, approximately 4 minutes each side for medium. Alternative: In a medium skillet over medium-high heat, sear the burgers 4 minutes on each side.
Grab the slices of Swiss cheese and place 2 slices over each grilled patty. Let the cheese melt for approximately 1-2 minutes. Optional: While the cheese is melting, slice the burger buns and place on grill for 1-2 minutes to form grill marks.
Grab a spoonful of Dijon mustard and spread it on each side of the bun.
Lay a handful of Arugula on one side of the bun then top with a patty.
To finish the burger, place the sautéed onions and mushrooms on the patty and top with the other half of the bun.