Melt the butter over medium heat in a large stockpot or Dutch oven. Add the onions and cook, stirring occasionally, until the onions soften and begin to turn translucent, about 5 minutes.

Add the garlic and cook, stirring frequently, until fragrant, about 1 minute. Do not let the garlic get too brown. Stir in the salt, pepper, thyme, and parsley. Cook for 1 minute.

Add broth, lemon juice, bay leaf, potatoes, and cauliflower. Bring to a boil, then reduce heat to medium-low and simmer for 35 minutes. The vegetables should be fork-tender.

Remove from heat. Remove bay leaf. Using an immersion blender, puree the soup until smooth and creamy. Or let cool slightly and then carefully transfer the soup to a regular blender and puree in batches (see Notes for instructions). If using, stir or blend in heavy cream.

Serve soup warm and top with chopped chives, croutons, and/or crumbled bacon, if desired.

Keep leftovers in a large covered container in the refrigerator for up to a week. To reheat, simply pour into a pot over medium heat and cook until warm.