Peel the tough outer layers of the lemongrass to expose the inner pale yellow layer. Trim 2-inches from the bottom, cutting away the bulb. Slice until the tops become tough. Add the pieces to a small pot over medium heat and add the stock and 1/2 cup water. Bring to a simmer and allow to reduce by half. Strain, reserving liquid.
Into a food processor add the cooked lemongrass, chilies, shallot, garlic, ginger, sugar, cilantro, mint, cumin, coriander, tamari, pepper, lime juice, and salt (if not using the fish sauce). Process into a paste.
Heat the oil in a wok or large saute pan over high heat; add the chicken and brown. Remove to a plate. Add a drizzle more oil and stir-fry the bell peppers and onions for 2 minutes. Stir in the paste for 1 minute, and then add the reserved liquid and stir. Return the chicken to the pan. Serve stir-fry on a bed of shredded lettuce.