Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper.
Wash the Hokkaido pumpkin thoroughly, slice it in half to scoop out the seeds, and then chop it into 1-inch cubes with the skin left on.
Cut the chicken breast into bite-sized pieces and finely mince the garlic cloves and fresh herbs.
In a large mixing bowl, combine the pumpkin cubes, chicken pieces, minced garlic, rosemary, and thyme.
Drizzle with olive oil and sprinkle with sea salt and black pepper, tossing everything until evenly coated.
Spread the mixture in a single layer on the prepared baking sheet, ensuring the pieces are not overcrowded.
Roast for 20 to 25 minutes, or until the chicken is cooked through and the pumpkin is fork-tender and caramelized.
Remove from the oven and immediately toss the hot roasted mixture with the baby spinach until it is just slightly wilted.