Preheat a panini style grill. Lightly grease it with nonstick spray.
Spread the mustard onto the bread. Layer with spinach, turkey, sun dried tomato, pepperoncini, cheese, and red onion.
Cook on the panini grill until golden brown and the cheese is melted. (Alternatively place on a baking sheet and cook in a 350 degree oven using a heavy oven safe skillet or pan to press the sandwich down until the cheese is melted and the sandwich is crispy).