Parsnip, Yam, and Watercress Chowder

Why on earth don't we eat parsnips on a regular basis? They are delicious simply braised and served with butter, but I absolutely love them in this soup.

2 Tbs butter
4 c 1/2 inch cubes peeled parsnips (about 4 large)
1 1/2 c chopped onion (about 1 large)
3 c low-sodium chicken broth1 c chopped and peeled Granny Smith apple (about 1 medium)
1 1/2 c peeled yam (red skin, orange flesh) cut into 1/2 inch cubes
1/8 tsp freshly ground nutmeg
1/8 c whipping cream
1 bunch watercress sprigs (tops of bunch of watercress)

Melt butter in a large pot over medium heat. Add parsnips and onion. Saute until onion softens, about 5 minutes. Add 3 c broth and apple. Cover and simmer until parsnips are tender, about 12 minutes. Puree 2 c parsnip mixture in a blender until very smooth. Return puree to pot. Add yam cubes and nutmeg. Cover and simmer until tender, about 12 minutes. Mix in cream and then watercress. Stir until watercress wilts, about 2 minutes. Thin chowder with more broth if desired. Season to taste with salt and pepper.