Make marinade and dressing: Combine brown sugar, smaller amount of fish sauce, lime juice, garlic, and chili heat of your choice in a medium bowl. Taste it; you want it to be salty and sour first, followed by sweetness and heat; adjust as needed.
Place steak in a sealable freezer bag. Pour in about ⅓ of the marinade and press out the air. Marinate in the fridge for about an hour or up to a day.
Whisk 3 tablespoons oil into the remaining marinade to make the dressing. Set aside.
Prepare other salad ingredients: Boil salted water and cook green beans for 2.5 minutes, then transfer to ice water to cool. Use the same water to cook the noodles according to package directions, then drain and set aside.
Get the grill hot and lightly coat grates with oil. Remove steak from marinade and grill for about 3.5 to 5 minutes per side, seasoning with salt and pepper. Let rest for 5 minutes before slicing thinly.
To serve: Place cherry tomatoes, cucumbers, and herbs in separate bowls. Serve noodles, steak, and dressing on the side. Assemble by starting with noodles, adding steak, vegetables, chili, dressing, and herbs.
