Preheat oven for 400 degrees and lightly grease a baking sheet.
Saute the spinach in a skillet with the olive oil for approximately 5 minutes. Spinach will be "squishy" looking.
Slice the chicken breast 4-5 times aka "hasselback". (cut enough that you don't cut it all the way through, making small pockets).
Cut the cheese block into small rectangles or squares - to fit into the chicken breast "pockets."
Place slices of cheese and a bit of spinach in each chicken breast "pocket".
*optional* Top the chicken breast with shredded cheese.
Bake for 25-30 minutes. Chicken is done when it temps out above 165 degrees.
Notes
Make sure to grease the baking sheet ! The cheese will melt and it will look messy. However, this also keeps the chicken moist and locks in lots of flavor. The less cheese you use, the less mess.Always use fresh/thawed chicken.This recipe is very versatile. Use whatever cheese you have on hand and you don't have to make it with spinach either. Sometimes I bread the chicken before slicing it and putting the cheese in it.