Pre-heat oven to 450°F.
Cut pumpkin into 2-inch pieces.
Combine pumpkin, onion, mushrooms, and garlic on a rimmed baking sheet.
Add oil and 2 teaspoons salt; toss to coat, then spread in a single layer.
Roast until pumpkin is tender when pierced with the tip of a sharp knife, about 30 minutes, rotating pan and tossing vegetables halfway through.
Let cool, then remove skins.
Transfer vegetables to a medium saucepan; heat over medium.
Pour in 2 cups stock.
Puree with an immersion blender until smooth.
With the blender running, slowly add remaining 3 cups stock, and puree until smooth.
Bring soup just to a simmer.
Remove from heat, and season with salt and pepper.
Cover to keep warm.
