Heat beurre noisette (or other fat) in a large saucepan over medium-high heat. Add shallots and cook, stirring frequently, until beginning to soften, about 5 minutes.
Add garlic and spices. Cook, stirring frequently, another minute, until fragrant.
Add lettuce. Stir constantly for 2-3 minutes to wilt the lettuce.
Add broth and plantain. Bring to a boil and the reduce heat to maintain a simmer, uncovered, for 20 minutes.
Puree the soup by putting the entire contents of the pot into your blender and blending on high for 1 minute (do this in batches if you have a smaller blender). An immersion blender can also be used, but will be harder to get that perfect creamy consistency.
