Mix mayonnaise and Dijon mustard in a small bowl and spread this mixture onto one side of each bread slice.

Assemble your panini: Place two slices of cheese on the unbuttered side of one slice of bread, add some cubed chicken, tomato slices, spinach leaves.

Add two more slices of cheese.

Top with another slice of bread, mayo-side down. Butter the outsides of your panini sandwich.

Preheat a panini press to medium-high heat. Place the sandwich on the grill and cook until the bread is toasted and the cheese is melted (about 3-5 minutes per side).

Remove from the panini press, let them cool for a minute, then slice and serve your delicious chicken panini!