Heat the butter in a large saucepan or dutch oven set over medium heat. Add the onion and cook, stirring occasionally, until soft and translucent, 5-7 minutes. Stir in the garlic and cook for 30-60 seconds, until fragrant.
Transfer the soup to a blender, add the mint, and puree until completely smooth, working in batches if needed. Return the soup to the pan and stir in the salt and pepper. Stir in the heavy cream, then add the lemon juice and adjust seasoning if needed.
1/2 cup packed fresh mint leaves, 1/2 teaspoon fine sea salt, 1/4 teaspoon freshly ground black pepper, 1/2 cup heavy cream, 1 tablespoon fresh lemon juice
Ladle into bowls. Garnish with creme fraiche and olive oil, if desired.