Preheat oven to 400°F (205°C). Then wash and dry your cauliflower, and cut it into small florets (no larger than 1 inch on any side), placing them into a large mixing bowl.
Mince the garlic and add it to the cauliflower, and then measure and add just 1 ½ Tablespoons of oil and 1 teaspoon of salt to the cauliflower & garlic in your bowl. Toss everything together until each floret has been coated with a thin layer of oil, salt, and garlic bits, then arrange them in a single layer on the silicone mat (placed on top of your baking sheet). Bake the mixture for 15 minutes.
Meanwhile, cut your chicken breasts into 8 to 12 strips. Once your timer goes off, take the tray of cauliflower out of the oven, then place your chicken strips on the tray with your cauliflower florets.
Drizzle your last 1 ½ Tablespoons of oil over the chicken, then sprinkle the sumac (if using), Italian seasoning, and rosemary onto botht he chicken and cauliflower bake, making sure each piece of chicken is evenly-coated.
Mix all the raw chicken pieces with the half-baked cauliflower so that they're both fully seasoned, then bake everything for 20 more minutes, until the centers of the chicken breast pieces reach 165°F/75°C. Once your timer goes off, simply plate and serve your baked chicken and cauliflower with a sprinkle of chopped basil (optional), preferably with a side of root vegetables, like roasted sweet potatoes or blistered carrots.