Scrub and pierce potatoes; place on a microwave-safe plate. Microwave, uncovered, on high for 15 minutes or until tender, turning once.
Sprinkle steak with pepper. Grill, covered, over medium heat or broil 4 in. from the heat for 7-10 minutes on each side or until meat reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 140°; medium-well, 145°). Let stand for 5 minutes.
Place tomatoes in a grill wok or basket. Grill, uncovered, over medium heat for 2-4 minutes or until tomatoes begin to soften. Keep warm.
In a large saucepan, heat artichokes and garlic over medium heat for 2 minutes. Add mayonnaise and 1/2 cup cheese; stir until cheese is melted. Stir in spinach and heat through.
With a sharp knife, cut an "X" in each potato; fluff pulp with a fork. Slice steak into thin strips. Top potatoes with steak, artichoke sauce and tomatoes. Sprinkle with remaining cheese.
Editor's Note: If you do not have a grill wok or basket, use a disposable foil pan. Poke holes in the bottom of the pan with a meat fork to allow liquid to drain.