The tomatillos, when roasted, round out their flavor into a wonderful, mellow heat. If you desire more, then add a jalapeno. If not, certainly leave it out. The dollop of creme fraiche will also help to tone down any fiery flavors. This soup is very flexible — feel like making it vegetarian? Leave out the chicken. Beans and corn kernels would work nicely as well.
Preheat the oven to 375°F. Roast the tomatillos (and jalapeno if desired) for about 30 minutes, flipping halfway. The peppers will be done when browned and soft to the touch. Let it cool for a few minutes, then place in a food processor with the cilantro and puree. If using the jalapeno, remove and discard seeds before pureeing. Set aside.
Meanwhile, in a medium saucepan over medium heat, heat the olive oil and cook the leeks, until softened. Add the garlic and season lightly with salt. Add the chicken stock, cumin and chicken, and bring to a boil. Let it cook until the chicken is cooked through, about 15 minutes. Remove the chicken and shred. Set aside.
Working in batches, put tomatillo puree and soup into a blender or food processor and blend until smooth. Return to the soup pot, bring to a boil and simmer. Adjust seasoning as necessary and put the chicken back into the pot and heat through.
Serve hot with a dollop of creme fraiche, additional cilantro garnish, and some crusty warm bread or tortilla strips.