If you're not into egg salad sandwiches, try pairing this Chilled Pea Soup with chicken salad or grilled cheese sandwiches. During cooler months, you can make this soup with frozen peas.
Make the soup: In a medium pot, melt butter over medium-high. Add onion and saute until translucent, 8 minutes. Add garlic and cook until fragrant, 1 minute. Add 1 cup water and simmer until onions are soft, 8 to 10 minutes. Add peas and cook until bright green, 4 minutes; season with salt and pepper. Transfer mixture to a blender and add buttermilk. Blend until smooth, then pour through a fine-mesh sieve into a large bowl, pressing on solids; discard solids. Refrigerate until cold, 1 hour, then season with salt and pepper.
Make the egg salad: Remove yolks from 3 eggs and discard. Coarsely chop eggs and mix with mayonnaise, mustard, and celery; season with salt and pepper. Make sandwiches with bread, egg salad, and arugula. Serve with chilled soup.
A pound of English peas in the pod yields about 1 cup shelled peas.