You won't miss the crust with this high-protein "pizza."
In a saucepan or skillet with a lid, heat 2 tbsp. oil, two turns of the pan, over medium to medium-high. Add the tomatoes (try our cherry tomato prep shortcut) and cook, stirring often, until softened a bit, 3 to 4 minutes. Add the garlic, oregano, crushed red pepper and a few torn basil leaves; cover and cook, shaking the pan occasionally, until the tomatoes break down, about 20 minutes. Uncover and cook, stirring occasionally, until the sauce thickens, about 5 minutes; season with salt and pepper.
Heat a large cast-iron skillet or griddle over medium-high, and preheat the broiler.
Working with 1 breast at a time, butterfly the chicken by cutting horizontally with a sharp knife almost all the way through. Open like a book and pound between sheets of plastic to about 1/4 inch thick. Season the chicken. Add 1 tbsp. oil, one turn of the pan, to the cast-iron skillet. Add 2 chicken breasts and cook until golden and cooked through, 2 to 3 minutes per side; transfer to a foil-lined baking sheet. Repeat with 1 more tbsp. oil, one turn of the pan, and the remaining 2 chicken breasts.
Top the chicken with the cherry tomato sauce, cheese and pepperoni. Broil until bubbly and brown, about 3 minutes. Top with a few more torn basil leaves.