Preheat your oven to 425°F and line a large rimmed baking sheet with parchment paper.
Cut the chicken breast into bite-sized cubes and place them in a mixing bowl with half of the olive oil, sea salt, and black pepper.
In a separate bowl, toss the cauliflower florets with the remaining olive oil, dried oregano, and a pinch of salt until well coated.
Spread the chicken and cauliflower across the prepared baking sheet in a single layer, ensuring they are not overcrowded to allow for crisping.
Roast in the oven for 18 to 22 minutes, flipping the chicken and tossing the cauliflower halfway through, until the chicken is cooked through and the cauliflower is caramelized.
While the sheet pan is roasting, mince the garlic and whisk it together with the lemon juice and lemon zest in a small bowl.
Remove the pan from the oven and immediately drizzle the lemon-garlic mixture over the hot ingredients, tossing gently to coat everything.
Garnish with finely chopped fresh parsley and serve immediately while hot.