Cut the chicken breast into ½-inch pieces.
Slice the garlic.
Cut the lemongrass into thin slices.
Heat a tablespoon of cooking oil in a skillet over medium-high heat.
Add lemongrass and garlic to the oil, and fry until the garlic turns brown.
Add chicken to the frying pan and stir together with the lemongrass and garlic. Continue frying over medium heat for 10 minutes.
Add the fish sauce, oyster sauce, sugar, and MSG to the frying pan and stir together.
Remove the chicken from the frying pan and set aside.
Wash and slice the cucumbers, red peppers, cilantro, and lettuce, which will be used as filling in the sandwich while the chicken is cooling off.
Slice the baguette in half, lengthwise.
Add the Cambodian spicy dipping sauce to the mayo and mix together.
Spread the mayo onto both surfaces inside the baguette.
Place the cucumbers, lettuce, and red peppers inside the baguette.
Add the chicken to the num pang sandwich.
Garnish the sandwich with cilantro.