Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Slice the Hokkaido pumpkin in half and scoop out the seeds, then cut into 1-inch thick wedges, keeping the edible skin intact.
In a large bowl, toss the pumpkin wedges with half of the olive oil, half of the sea salt, and half of the black pepper until evenly coated.
Mince the garlic and combine it in a small bowl with the remaining olive oil, dried oregano, and the rest of the salt and pepper.
Rub the garlic-herb mixture all over the chicken breast and place it on one side of the prepared baking sheet.
Arrange the pumpkin wedges in a single layer on the other side of the baking sheet.
Roast for 20 to 25 minutes, or until the chicken reaches an internal temperature of 165°F and the pumpkin is fork-tender with golden edges.
While roasting, whisk together the tahini and lemon juice in a small ramekin, adding a teaspoon of warm water at a time until it reaches a pourable consistency.
Allow the chicken to rest for 5 minutes before slicing it into strips.
Plate the roasted pumpkin wedges and sliced chicken, then finish the dish with a drizzle of the tahini sauce and a sprinkle of chopped fresh parsley.