Preheat the oven to 400F / 200 C.

Cut the hard stem off the butternut squash and cut the squash in half lengthwise. Using a teaspoon, scoop out and discard the seeds.

Brush the flesh side of the squash with olive oil and one tablespoon of balsamic vinegar. Season to taste with salt and pepper. Bake, covered, until the squash is soft and the skin somewhat charred, about 45 minutes to an hour. Remove and cool slightly.

In the meantime, heat 2 tablespoons of olive oil in a large skillet over medium heat and sauté the leeks and onion for about 15 minutes. Add the remaining tablespoon of balsamic and continue cooking until lightly caramelized. Stir in the garlic then add the spinach, in batches if necessary, and toss all together until the spinach wilts and there is no liquid left in the frying pan. Stir in the dill. Season to taste with salt, pepper, and nutmeg. Stir in the feta.

Before serving: Lower the oven temperature to 350F / 170C. Fill the butternut squash with the spinach mixture and sprinkle with pumpkin seeds, if desired. Cover and place back in the oven. Bake all together for 10 minutes. Remove and serve.