Melt butter in a large pot over medium heat. Add chopped onion and sauté for 3–4 minutes until translucent. Stir in garlic and cook for 30 seconds until fragrant.
Add broccoli florets and broth. Bring to a boil, then reduce heat and simmer for 10–12 minutes until broccoli is tender.
Use an immersion blender to purée the soup until smooth (or carefully transfer to a blender in batches).
Return soup to low heat. Stir in cream, salt, pepper, and nutmeg. Simmer for 3–4 minutes (do not boil).
Add lemon juice for brightness if desired. Ladle into bowls and garnish with cheddar cheese or a drizzle of cream.
