Preheat a panini grill, George Foreman style grill, or if using the oven, preheat to 350°F.
Spread the mayo on both slices of bread, layer with spring mix, roast beef, cheese, and tomato.
Lightly grease the panini style grill with the non-stick spray. Grill the sandwich until the cheese is melted and the bread is golden brown. (Alternatively, place in the oven on a baking sheet until the same result occurs.)