In your slow cooker, add the chopped onion, minced garlic, diced celery, chopped zucchini, and the optional Yukon Gold potato if you're using it. Pour in the vegetable broth, sprinkle with kosher salt and freshly ground black pepper, add dried thyme, crushed red pepper flakes, and the bay leaf. Drizzle with melted butter or olive oil, and give everything a good stir to combine.

Cover the slow cooker with its lid and set it to cook on Low for 6-7 hours or on High for 3-4 hours. The soup is ready when the zucchini is very tender and easily breaks apart when stirred.

Once the soup has finished cooking, carefully remove the bay leaf and discard it. Use an immersion blender to puree the soup until it achieves a creamy and smooth consistency.

Incorporate the heavy cream, Greek yogurt, or coconut milk into the blended soup along with the fresh lemon juice and optional grated Parmesan. Blend briefly to mix all ingredients.

Evaluate the soup's thickness; if it’s too thick for your liking, gradually stir in additional broth until it reaches your desired consistency.

Ladle the Crockpot Creamy Zucchini Soup into bowls, and finish by garnishing with fresh herbs like basil, chives, or parsley.