Heat the panini maker. If you don't have a panini maker, you could use a pan with a little bit of butter and brown the sandwich on each side.
Lay out 4 slices of the bread on your work surface. Divide the fajita mixture in 4 and top each slice with the chicken fajita mixture. Place a slice of cheese on top of the chicken fajitas then finally the other slice of bread.
Place the sandwich in the panini maker and grill for about 3 or 4 minutes, or until cheese has melted and you have grill marks.
Serve with salsa or sour cream if preferred.
Total time includes time for making the chicken fajitas.