Heat oil in a large saucepan or stock pot, on medium heat. Add onion and garlic (and red pepper flakes, if desired). Cook for 3-5 minutes, until tender and fragrant.
Add squash and cook for 2-3 minutes. Add stock, salt and pepper and bring to a boil. Cover pot, reduce heat and simmer for 30 minutes or until squash is tender.
Puree soup in blender, food processor or with an immersion blender. Return to heat. Stir in evaporated milk. Taste and adjust salt and pepper, if necessary.
Use already peeled and cut up squash for easy and quick preparation.