Remove leaves from cauliflower, core, and cut into bite-sized florets. Zest lemon, halve, and cut two thin rounds off ends. Juice remaining lemon. Mince garlic. Halve tomatoes. Stem and mince parsley. Pat chicken breasts dry, and season both sides with 1/4 tsp. salt and a pinch of pepper.

Toss cauliflower, 1 Tbsp. lemon juice (reserving remaining for stuffing), 1 Tbsp. olive oil, half the garlic (reserving remaining for stuffing), and 1/4 tsp. salt and a pinch of pepper on prepared baking sheet. Spread into a single layer and roast until tender and caramelized, 22-25 minutes. While cauliflower roasts, prepare spinach stuffing.

Heat 1 tsp. olive oil in a medium oven-safe non-stick pan over medium heat. Add remaining garlic and cook until aromatic, 30-60 seconds. Add spinach and tomatoes and cook until spinach has just wilted, 1-2 minutes. Stir in half the Parmesan (reserve remaining for garnish), lemon juice (to taste), and a pinch of salt and pepper. Transfer stuffing to a plate and set aside to cool. Wipe pan clean and reserve.

Lay chicken on a cutting board. Hold knife blade parallel to the board and carefully make a lengthwise cut through the center of the meat, using your free hand to steady meat. Stop short of opposite edge of chicken so it remains in one piece. Open as you would a book. Stuff chicken with spinach stuffing. Melt butter in pan used to cook spinach over medium-high heat. Add stuffed chicken to hot pan and cook undisturbed until golden brown, 2-3 minutes.

Flip chicken and top each with a lemon round. Place pan in oven and roast until chicken reaches a minimum internal temperature of 165 degrees, 9-11 minutes. Remove pan from oven.

Place a stuffed chicken breast on a plate and serve with roasted cauliflower. Garnish with parsley, remaining Parmesan, and lemon zest (to taste).