WHISK together ingredients for dressing in a small bowl. Set aside.
ADD spring mix, tomato and fennel to a large bowl. Toss with dressing. Arrange salad in a bowl or serving dish. Scatter red onion slices.
HEAT a skillet over high heat. Season steak with salt and pepper. Once pan is very hot add steak. Cook 3-4 minutes. Turn steak over and add butter and garlic to pan. Use tongs to move steak around in butter and garlic. Cook an additional 3-4 minutes. Allow steak to rest 5 minutes. Thinly slice steak and place over salad.