Add classic Italian Sub fillings to a grilled panini sandwich for a quick, easy and delicious meal.
Thinly spread mayonnaise on one side of all four bread slices. Heat grill pan over medium-high heat and add 2 slices of bread, mayonnaise side down. (If the pan is not cast iron, spray with non-stick spray first).
Add lettuce, tomato, red onion, salami, ham, pepperoni and provolone cheese on top of the bread slices in the pan. Sprinkle with salt and pepper, if desired. Top assembled sandwiches with remaining 2 bread slices, mayonnaise side up.
Place a piece of aluminum foil on top of the sandwiches. Place a cast iron skillet on top of the foil, applying even pressure to the top of the assembled sandwiches.
When the bread on the bottom is golden brown, carefully flip each sandwich and repeat until the other slice of bread is also evenly browned. Serve immediately.
For best results, soak the red onion slices in water for at least half an hour to soften before serving. This will also yield a milder flavor. Remove onion from water and pat dry before placing on the sandwich.