Heat butter in a large dutch oven. Add onion, celery, and carrot. Let cook 5-7 min or until beginning to soften. Add garlic and thyme. Cook 2 minutes.
Stir in water and bay leaf. Bring to a boil. Reduce heat and let simmer 10-12 minutes.
Add corn and zucchini. Return to boiling.
Let simmer 10-12 more minutes. Remove bay leaf.
Transfer 1 1/2 c. of the chowder to a blender and puree until smooth.
Return pureed chowder to pot. Stir in milk and heat just until hot. Serve with peppers and parsley, if desired.