Combine everything together in a large glass bowl, cover with plastic wrap and let it marinate for at least a couple of hours, even better over night.
Pour the leftover marinade and dark brown sugar into a small sauce pan over medium-high heat. Let it come to a boil whisking it for about several minutes until it thickens just enough to glaze the back of a spoon. Mix in about 1/2 tablespoon of olive oil and set aside.
Assemble the skewers with the chicken, onion, bell peppers, and tomatoes by alternating them.
Put the skewers on the grill, brush the marinade generously on top and close the grill for about 3 minutes to get a char started. Turn the skewers about every 2-4 minutes and brush more marinade each time for a total grilling time of about 15 minutes give or take.
Remove the skewers onto a clean plate and if there’s any marinade leftover you can always place the remaining on top. Garnish the top with some chopped cilantro.