Flavor Base: Melt butter in a large pot over medium heat. Add onion and garlic, cooking until onion is soft and fragrant, about 5 minutes.
Soupify: Add 3 cups of the peas (reserve other 1 cup), mint, and 2 cups of the broth. Cover and bring to a gentle simmer for 10 minutes.
Blend: Using a handheld immersion blender (or by carefully transferring to a countertop blend), blend the soup until smooth. (If using a countertop blender, leave the lid ajar to allow steam to escape).
Assemble: Return soup to the pot and add remaining 1 cup of peas and remaining 2 cups of broth. Taste and add salt and pepper, as needed.
Serve: Serve hot, optionally serves with parmesan cheese and crème fraîche.
Notes
Store leftovers in an airtight container in the fridge for 4 to 5 days, or freeze for up to 2 months.