Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Dice the chicken breast, sweet potato, and Hokkaido squash into uniform 1-inch cubes to ensure even cooking.
Finely mince the garlic clove and strip the rosemary leaves from the stem, chopping them finely.
In a large mixing bowl, combine the chicken, sweet potato, and squash cubes.
Drizzle with olive oil and sprinkle with the minced garlic, chopped rosemary, sea salt, and black pepper.
Toss everything thoroughly until the chicken and vegetables are well-coated in the oil and herbs.
Spread the mixture in a single layer on the prepared baking sheet, ensuring pieces are not overcrowded.
Roast for 25 to 30 minutes, tossing halfway through, until the chicken is cooked through and the vegetables are fork-tender and golden brown.