Heat oil in a pressure cooker over medium-high heat. Saute onion until translucent but not browned, about 5 minutes. Add pumpkin, stock, sweet potatoes, apples, garlic, cinnamon, nutmeg, black pepper, cumin, ginger and salt.
Close and lock the lid. Bring to high pressure according to manufacturer's instructions, 10 to 15 minutes. Reduce heat to medium-low; cook until squash and sweet potatoes are fork-tender, about 15 minutes.
Release pressure carefully using the quick-release method according to manufacturer's instructions. Unlock and remove lid. Liquefy soup using an immersion blender. Stir in milk and cream. Serve with a dollop of yogurt on top.
Use homemade vegetable stock and/or yogurt if possible.