Pre-heat the oven to 400°F.
Dress the squash with EVOO to coat lightly and season with salt, pepper and a little freshly grated nutmeg. Toast to golden at the edges and very tender, about 25 minutes.
In a soup pot, heat a drizzle of EVOO over medium to medium-high heat. Add the butter and melt; add the carrots, celery, onion, garlic, citrus zest, salt and pepper. Cook, partially covered, to soften the vegetables, 10 minutes. Add the saffron, stock or water and a drizzle of honey. Bring to a boil, then reduce to a simmer to soften the vegetables, 20 minutes.
Puree with an immersion blender. Add the roasted squash and blend to combine, thinning with extra water or stock if necessary. Adjust the seasonings, to taste. Top with toasted pistachios and pumpkin seeds.