Heat butter in large pot or Dutch oven over medium heat. Add leek, and sauté 4 to 5 minutes, or until softened. Add butternut squash, and season with salt and pepper, if desired.
Add cauliflower, and sprinkle with flour. Stir in broth, milk, bay leaf, and thyme sprigs; season with salt and pepper, if desired. Bring to a boil, then reduce heat to medium-low. Simmer 10 minutes, or until vegetables are tender.
Stir in corn and lemon zest, and cook 2 to 3 minutes more. Remove thyme sprigs.