In a small bowl, whisk the honey mustard and oil. In another small bowl, stir the chili powder, salt, pepper, paprika, garlic powder and thyme, if using. Rub the mustard mixture all over the flank steak, then coat with the spices.
Preheat a grill or grill pan over high. Grill the steak until charred in spots, turning once, about 3 1/2 minutes per side for medium-rare.
Transfer the steak to a cutting board and let rest 5 to 10 minutes. Thinly slice the steak against the grain.