Remove dark-green outer leaves of romaine lettuce, and coarsely chop (to yield 8 cups). Reserve remaining lettuce for another use.
In a large pot, melt butter over medium heat. Add onion, 1 teaspoon salt, and 1/4 teaspoon pepper. Cook, stirring occasionally, until onion is softened, about 5 minutes. Add lettuce; stir until just wilted, about 2 minutes.
Add chicken broth and 2 cups water; bring to a boil. Add peas, and simmer until bright green, about 4 minutes. Remove from heat.
Stir in mint. Season again with salt and pepper. Using an immersion or regular blender (work in batches), puree soup until smooth. If necessary, reheat soup over medium-low heat. Serve garnished with croutons.