Preheat oven to 400°F (200°C). Place squash in foil-lined 15 x 10 x 1-in. (38 x 25 x 2.5 cm) baking pan. Drizzle with oil. Sprinkle with sage, salt and pepper. Toss to coat. Roast 30-35 minutes or until tender, stirring occasionally.
Meanwhile, in large saucepan, heat oil and 1 tbsp (15 mL) butter over medium heat. Add onion and garlic. Cook and stir 3-4 minutes or until softened. Reduce heat to medium-low. Cook 30-40 minutes or until deep golden brown, stirring occasionally. Stir in salt, pepper flakes and pepper.
Add water, sweet potato and carrot to saucepan. Bring to boil. Reduce heat. Cook, uncovered, 10-15 minutes or until vegetables are tender. Add squash mixture and remaining butter to soup. Puree soup using an immersion blender. Or, cool soup slightly and puree in batches in a blender. Return to pan and heat through.