Make the yogurt-honey marinade: in a medium bowl whisk together the Greek yogurt, 4 tablespoons olive oil, minced garlic, lemon zest, lemon juice, 1 teaspoon kosher salt, 1/4 teaspoon black pepper, 2 teaspoons Greek seasoning, 2 teaspoons ground fennel, honey, and 1 tablespoon chopped parsley until smooth and combined.
Marinate the chicken: place the chicken breasts in a large zipper bag, pour in half of the marinade, press out excess air, seal, and squish the bag so the chicken is fully coated. Refrigerate 1–12 hours.
Reserve the remaining marinade: transfer the other half of the yogurt-honey sauce to an airtight container and refrigerate for serving.
Prepare the cauliflower: cut the cauliflower into pieces no more than 1/2 inch thick, place in a bowl, add 2 tablespoons olive oil, 1/4 teaspoon kosher salt, 1/4 teaspoon black pepper, and 1/4 teaspoon ground fennel; toss gently to coat and refrigerate until ready to roast.
Preheat and prepare the sheet pan: heat the oven to 350°F. Line a large baking sheet with aluminum foil and spray with nonstick spray. Cut a piece of parchment slightly larger than the chicken and set aside.
Arrange on the sheet pan: remove chicken, reserved sauce, and cauliflower from the fridge. Place the marinated chicken on one end of the baking sheet and spray one side of the parchment then place it sprayed-side down over the chicken, tucking around the pieces. Spread the cauliflower in a single layer on the other side of the sheet without crowding.
Roast: bake at 350°F for 20–30 minutes, until the internal temperature of the chicken reaches about 155°F.
Broil for color: remove the parchment and switch the oven to broil. If using tomatoes, add them around the cauliflower now. Broil 2–4 minutes to brown the chicken and caramelize the cauliflower, watching carefully.
Finish cooking: continue until the chicken registers 162–165°F, then remove the chicken and loosely tent with foil. If cauliflower needs more color, leave it under the broiler a little longer.
Rest and serve: transfer the sheet pan to a cooling rack and let the chicken rest 2–3 minutes. Garnish with remaining chopped parsley and serve with the reserved yogurt-honey sauce.