Prepare vinaigrette: In a medium bowl, whisk together first 5 ingredients (shallot through salt and pepper). Add tomatoes; toss to combine then let marinate while you cook the mushrooms.
Preheat outdoor grill or a large skillet, brushed with the ghee, to MEDIUM. Grill mushrooms and steaks for 5 to 7 minutes or until fork-tender and beef is cooked to the desired doneness. Serve with tomato vinaigrette on top of bed of spinach.