In a bowl, mix together creamed spinach, Swiss cheese, Parmigiano Reggiano, and bacon. If the mixture seems too thick, add a little bit of half and half to thin. It should be creamy but hearty in texture so that it doesn't run out of the chicken breasts. Season to taste with salt and pepper.
Use a sharp knife to make pockets in each chicken breast. Insert the tip of the knife into the thickest part of the side of the breast, about 1/2-inch from the end. Cut about 3/4 of the way into the breast, extending the slit about 1/2- to 1-inch from the tapered end of the breast.
Fill each pocket with a quarter of the filling (about 1/3 cup). Close the breasts and secure with a skewer so they don't open during cooking.
Season the stuffed chicken breasts with salt, pepper, and a pinch each of paprika and garlic powder.
Heat oil in a 12-inch skillet over medium-high heat until shimmering. (I use cast iron.) Add the chicken breasts, top side down, and sear for 5-7 minutes. Flip the breasts, cover the pan, and continue cooking for 5-7 minutes more until the centers and the thickest part of the breast read 165 degrees F on an instant read thermometer. (Cooking time depends on the thickness of the breasts.)
Remove from the pan, tent with foil, and let rest for 5 minutes before slicing and serving.
Make the creamed spinach recipe up to 2-3 days in advance and store, covered, in the refrigerator.