Preheat the oven to 180 degrees Celsius (350 Fahrenheit). Cut the pumpkin in half and remove the seeds. Lightly coat the cut surface of the pumpkin with olive oil and season with sea salt and place on a tray lined with baking paper cut side down.
Remove the skin from the onion and cut in half. Toss the onion, potatoes and garlic cloves (keep the skin on) in the remaining oil and season with sea salt. Place on the baking tray with the pumpkin and roast until the pumpkin and potatoes are tender. Remove the garlic and onion from the tray and set aside if they cook before the pumpkin and potatoes.
Cut the potatoes in half and scoop out the creamy flesh and place in a blender along with the onion. Squeeze the garlic out of their papers and place in the blender. Finally, scoop the pumpkin flesh from the skin and place in the blender. Add the vegetables stock (you might need to blend the soup in two batches depending on the size of your blender), honey and rosemary and blend until smooth. Taste and adjust seasonings as necessary.
Place in a saucepan over a medium low heat and reheat before serving.
Spice things up a bit with the addition of a ¼ to ½ tsp ground cinnamon and ginger.