Season the chicken breasts on both sides with salt and pepper and cook them in a sauté pan with 1 tablespoon of olive oil over medium-high heat until golden brown on each side and cooked throughout, about 5 to 7 minutes per side.
Remove the chicken and roughly chop. Set aside.
Next, add the cauliflower to a food processor and pulse on high speed until it resembles the size of rice. Remove and gently saute in a large frying pan over medium heat with 1 tablespoon of olive oil for 4-6 minutes.
Fold the chopped chicken, tomatoes, 8 ounces of shredded mozzarella, spinach, and salt and pepper into the cooked cauliflower until combined.
Transfer the mixture to a 13x9 casserole pan and top off with remaining 8 ounces of shredded mozzarella cheese, sliced mozzarella cheese, parmesan cheese, tomatoes and basil leaves, and bake in the oven at 375° for 35 to 45 minutes or until the cheese is golden brown on top.
Drizzle some pesto over the top of the casserole and serve!