In a bowl, combine fish sauce, brown sugar, lime, and garlic. Pour half of the marinade in a resealable bag and add the steak. Marinate in the fridge for at least 30 minutes or overnight. Reserve the remaining sauce and add red onion.
Preheat the grill and oil the grill grates. Grill the steak for 3-5 minutes on each side, or until done to the degree of doneness you prefer. Transfer to a cutting board and let it rest for 10 minutes, covered with aluminum foil. Thinly slice the steak and pour the remaining sauce.
In a bowl, mix bok choy with sesame oil and salt. Lay the stalks on the preheated grill, and cook until the bok choy stalks show grill marks and the leaves are crisp at the edges.
Peach salsa: combine all ingredients in a bowl.