In a soup pot over medium-low heat, warm the olive oil.
Add the celery, onion, garlic, and bay leaf and sauté until soft, about 5 minutes.
Add the peas and 4 cups water and bring to a simmer over medium-high heat; simmer for 10 minutes.
Reduce the heat to a gentle simmer and cook for another 20 to 25 minutes, until the peas are completely tender.
Towards the end of cooking, add salt and pepper to taste. Remove the bay leaf.
Using an immersion blender (or a regular blender), puree the soup until smooth.
If the soup seems too thick, add some hot water to thin it out to the desired consistency.
Taste and adjust the seasonings, adding lemon juice to taste.
Serve hot, garnished with prosciutto, if desired, and black pepper.
