Onions: Provide a sweet and savory base flavor (can vary based on onion type eg yellow, white, red)
Carrots: Add sweetness and earthiness.
Celery: Adds a savory note and depth.
Leek: Adds a mild, sweet onion flavor.
Garlic: Adds a pungent, savory flavor.
Bay leaf: Adds a subtle herbal note (lots of people think bay leaves add very little to most things but fresh bay leaves really help)
Parsley stems: Adds freshness and a herbaceous flavor.