I recently took a Thai cooking class at the Institute of Culinary Education in Manhattan and ever since I've been trying to work traditional Thai flavors into my cooking. Classic Thai food has an amazing mix of hot, salty, and sour and I tried to work that into this flavorful soup. The soup pops in your mouth, with bright highs of all the flavor combinations.
The lemongrass and kaffir lime leaves might be hard to find in your area but they can both be ordered online from Amazon.com and stored in the refrigerator for months. I used peas, carrots, mushrooms, and collard greens but go ahead and substitute anything tasty you have around the house. You can also use pork, beef, or shrimp instead of the chicken if you prefer.
Preheat the water bath to 141°F (60.5°C).
Salt and pepper the chicken and place in the sous vide bag. Seal the bag and cook the chicken for 2 to 4 hours, until pasteurized.
Cut off the top of lemongrass, using only the bottom 4 to 5 inches and cut into large chunks. Cut or tear the kaffir lime leaves into strips.
Place the chicken stock, lemongrass, and kaffir lime leaves in a pot and bring to a simmer. Let the flavors infuse for at least 15 minutes, and up to 30 minutes. Strain the lemongrass and kaffir lime leaves out of the soup.
Stir in the Tom Yam sauce and fish sauce and bring to a boil. Remove from the heat. Right before serving stir in the lime and add additional lime juice, salt, or Tom Yam sauce to balance the soup.
Take the chicken breasts out of the water bath and remove them from the bag. Dry them off thoroughly using paper towels or a dish cloth. Quickly sear the chicken for 1 to 2 minutes per side, until just browned, then remove from the heat and cut into strips.
Place some of the sautéed vegetables in the bottom of a bowl and top with the buckwheat noodles. Add the sliced chicken and a ladle or two of the soup. Sprinkle the cilantro on top then serve.