Heat oil in large saucepan over medium heat. Add onion and cook 3 to 4 min., until starting to soften. Stir in squash, celery, garlic and pepper. Pour in vegetable broth. Bring to boil over high heat, then reduce heat to low. Simmer, uncovered, 25 to 30 min., until squash is fork-tender.
Use an immersion blender to purée soup until smooth. Reheat on low before serving.
After puréeing the soup, stir in 1 tbsp (15 mL) finely grated ginger and 1 tsp (5 mL) hot pepper flakes for a warm, spicy kick.