Preheat your oven to 425°F and line a large rimmed baking sheet with parchment paper for easy cleanup.
Dice the chicken breast into uniform 1-inch pieces and place them in a large mixing bowl.
Add the cauliflower florets to the bowl with the chicken pieces.
In a small jar or bowl, whisk together the olive oil, lemon zest, lemon juice, minced garlic, rosemary, thyme, salt, and pepper.
Pour the lemon-herb dressing over the chicken and cauliflower, tossing well to ensure every piece is evenly coated.
Spread the mixture onto the baking sheet in a single layer, making sure not to crowd the pan so the cauliflower roasts instead of steams.
Roast for 20 to 25 minutes, giving the pan a good shake halfway through, until the chicken is cooked through and the cauliflower is tender with charred, crispy edges.