In a large bowl, combine the brown sugar and soy sauce. Squeeze the garlic through a press held over the bowl, scraping off and adding the garlic that sticks to the bottom of the press. Whisk in the spices, lemon juice, and orange zest, and gradually whisk in the olive oil.
Set aside 2 Tbs. of the marinade. Toss the beef cubes in the bowl with the rest of the marinade and marinate the meat at room temperature, covered, for 30 minutes, (or for up to 8 hours in the refrigerator), turning the meat several times in the marinade.
Prepare a medium-hot charcoal fire or heat a gas grill.
Skewer the cubes, leaving about half an inch of space between each cube so they’ll cook all around. Grill the meat, turning the skewers to brown on all sides to the point of slight charring, for about 8 minutes, depending on your grill, for medium to medium rare.
Push the meat off the skewers and onto plates, drizzle with the reserved 2 Tbs. marinade, and garnish with cilantro or mint sprigs and a spoonful of yogurt, if you like.