Preheat a panini grill or George Foreman style grill.
Mix the mayo, dill, and lemon juice in a bowl. Season with salt and pepper and then spread onto both halves of the flatbread and top each half with cheese. Layer with spring mix, roast beef, and tomato seasoned with salt and pepper.
Lightly grease the panini style grill with non-stick spray and press the sandwich down. Cook until the bread is crispy and the cheese is melted. (Alternatively, lightly grease a non-stick pan over low-medium heat with non-stick spray and cook both sides of the sandwich until golden brown and melted while covering with a lid.)