In a small bowl, combine the fish sauce, lime zest, lime juice, brown sugar, garlic and jalapeño. Pour half of the sauce / marinade over the flank steak and marinate at room temperature for at least 30 minutes, alternatively also overnight in the refrigerator (up to 24 hours). Before cooking, let the meat come back to room temperature.
Mix the cucumber, radishes and shallots in a salad bowl and add enough marinade.
Heat up the grill and allow the meat to drain (can also be dabbed with paper towels). Grill for between 3 - 5 minutes per side over direct heat (depending on whether the meat is to be rare or medium). Then wrap in aluminum foil and let rest for approx. 5 - 10 minutes.
Now cut the meat into thin slices and arrange on a plate with the rice or rice noodles. Add the cucumber salad. The meat can be garnished with sesame seeds or the roasted peanuts as well as mint leaves or coriander. Use the remaining marinade as a sauce if you like.
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