Prep the oven and pan: Preheat the oven to 425°F and line a sheet pan with parchment paper for easier cleanup.
Slice the chicken: Using a sharp knife, cut evenly spaced slits across each chicken breast, leaving about ¼ inch intact at the bottom so the chicken stays together. Aim for cuts about ½ inch apart. I like to take my time with this step because it makes stuffing the chicken much easier later on.
1½ boneless
Season the chicken: In a small bowl, mix the olive oil, salt, black pepper, garlic powder, sweet paprika, and Italian seasoning. Brush the mixture all over the chicken, making sure some gets inside the slits too. Don't brush everything, leave some for the veggies. I find this much less messy than sprinkling the seasonings directly over the chicken.
1 tablespoon extra virgin olive oil, 1½ teaspoon kosher salt, ½ teaspoon black pepper, ½ teaspoon garlic powder, ½ teaspoon sweet paprika, 1 teaspoon Italian seasoning
Stuff with vegetables: Gently fan open the cuts and tuck slices of zucchini, tomato, and red onion into each slit. Try not to overstuff them, or the vegetables may slide out during baking. brush with remaining seasonings.
1 small zucchini, 1 vine tomato, ½ red onion
Add the cheese: Sprinkle the mozzarella and Parmesan evenly over the top of each chicken breast. Don't worry if some cheese falls onto the sheet pan; those crispy cheesy bits are often the first thing to disappear at my house.
½ cup freshly grated mozzarella cheese, ¼ cup freshly grated Parmesan cheese
Bake until juicy: Bake for 17–20 minutes, or until the thickest part of the chicken reaches 165°F on an instant-read thermometer.
Broil for extra color (optional): For a more golden, bubbly cheese topping, broil for 30–60 seconds at the end. Keep a close eye on it because the cheese can brown quickly.
Garnish and serve: Finish with freshly chopped basil and serve warm. I usually add an extra sprinkle of Parmesan just before serving because, in my opinion, there's no such thing as too much cheese in this recipe.
Chopped fresh basil