Stir the pesto, Parmesan, and red pepper flakes in a small bowl.
Spread butter on one side of each bread slice.
Flip the bread and spread the pesto mixture on the unbuttered sides.
Layer mozzarella, provolone, tomato, and spinach over half the slices. Cap with remaining bread, buttered side out.
Warm a large nonstick skillet over medium-low. Cook sandwiches 3–4 minutes per side until deeply golden and cheese melts.
Rest 1 minute on a board, then slice and serve hot.
Notes
Variation: Swap half the mozzarella for sharp white cheddar for a bolder bite. Air fryer option: Cook at 370°F for 6–8 minutes, flipping once. To store, cool completely, wrap, and refrigerate up to 2 days; re-crisp in a skillet.This recipe is an original creation inspired by classic Pesto Grilled Cheese Sandwiches flavors. All ingredient ratios and instructions are independently developed.