In a large soup pot on medium heat, saute the onions in the butter and oil for 3 to 4 minutes. Stir in the celery, cover, and cook until just soft, stirring occasionally. Add the carrots, potatoes, water or stock, thyme, bay leaf, salt, and pepper and bring to a boil; then reduce the heat, cover, and simmer until the vegetables are just tender, about 5 minutes. With a strainer or slotted spoon, remove about 1 1/2 cups of the cooked vegetables and set aside in a blender or food processor. Add the green beans, bell peppers, and zucchini to the soup pot and cook until the green beans are tender, about 5 minutes. Stir in the peas and parsley, simmer for 2 more minutes, and then remove from the heat. Discard the bay leaf. Puree the reserved vegetables with the milk and cheeses to make a smooth sauce. Stir the sauce into the soup and gently reheat. Serve hot.