Potage Fontange (Mixed Vegetable Soup)

This is a soup which was eaten at Giverny, Monet's country home where grows to this day the garden that inspired so many of his paintings. I've leaped into the book Monet's Table once again and tried a couple more recipes. I've made substitutions to this soup just because some of the ingredients are a little challenging to find here - not impossible, just not at the grocery store nearby. I've indicated the substitutions as well as the original ingredients for the sake of keeping a record - my additions are in parentheses and italicized.

Part 1 - the peas

2 cups dried green or yellow (split) peas

Part 2 - the soup

2 Tbsps (28gr) of butter

1 onion, thinly sliced

2 leeks (12oz/340gr) white parts only, thinly sliced

1⁄2 lb sorrel (or spinach with lemon juice), trimmed and shredded

1 head iceberg (or romaine) lettuce, trimmed and shredded

3 sprigs chervil (or French tarragon or parsley), finely chopped

2 medium potatoes, peeled and cut in half (or quarters for a piece in each bowl)

6 cups rich beef broth (or any broth) or water

1⁄2 tsp salt

1⁄2 tsp pepper
Part 3 - the cream

½ cup (4oz/114gr) butter, at room temperature

2 egg yolks

1 cup crème fraîche or heavy cream

Bread to accompany the soup

1. The peas: Soak the dried peas in water for 2 hours (I put it in a pot and left it on the stove at the lowest temperature, just to keep the water hot but not boiling to make this go faster) then drain.
2. The soup: In the meantime, melt 2 Tbsps of butter in the soup pot. Add the onion, leeks, sorrel (or spinach, not the lemon juice yet), lettuce and chervil. Cook over low heat until the veg are well coated; do not let them brown.
3. Add the peas and potatoes, give it a good stir, then add the broth. Season with salt and pepper and bring to a boil. Cover and simmer over very low heat for at least 2 hours or until the peas are well soft.
4. (If using spinach, add the lemon juice now). At this point you have two choices: 1) strain the soup (using a vegetable mill or forced through a sieve using a wooden spoon) or 2) keep it chunky where it's recommended you break up the potato with the back of a spoon and mix it in. Either way, you'll then need to make the cream, and for this you must...
5. The cream: The purpose of this step is to enrich the soup with a classic French liaison — it will give the finished soup a velvety, rounded quality that sets it apart from a plain pea potage. 
1. Beat the softened butter with a wooden spoon until it is completely smooth with no lumps. 
2. Beat in the crème fraîche or cream a little at a time until fully incorporated, then beat in the egg yolks one at a time. The mixture should look uniform and creamy.
3. Place this cream either in a warmed soup tureen or divide it among individual bowls. Your soup must be very hot when you pour it on top — ladle a small amount of the hot soup over the cream first, stirring gently to begin warming it and loosening it, then pour or ladle the rest of the soup over slowly, stirring as you go. This gradual approach warms the egg yolks gently without scrambling them, and brings the whole thing together into a rich, unified soup.
6. Enjoy with crusty bread.