Preheat oven to 500.

Cut peppers into halves or quarters. Take out stems and seeds. Roast in oven for about 15 minutes until charred. Cover tightly with foil. Let cool for 10 minutes. Remove skins. Check out this tutorial for additional tips.

Slightly flatten chicken with a mallet if needed.

Place chicken on panini grill. Season with Kosher salt, freshly ground pepper and garlic powder. Grill until done. Can also be baked in oven. Slice chicken into strips.

Combine mayonnaise and Dijon mustard. Set aside.

To make the panini, spread mayonnaise/Dijon spread on one side of all the pieces of bread. On four of the pieces, arrange 1/4 of the chicken slices, 1/4 of the pepper slices, a slice of pepper jack cheese and a handful of spinach. Top with remaining bread slices, pesto side down.

Brush the outer sides of each sandwich with olive oil.

Heat panini griddle or skillet over medium. Grill sandwiches until browned on one side; turn and grill until brown on other side.