Set ribs out to come to room temperature.
In a bowl combine, brown sugar with chile powder, garlic, onion flakes, rosemary, smoked paprika and salt.
Add sugar and spices to a saucepan with mustard and mix well.
Cook over medium heat, stirring constantly just until sugar is melted, about 8 minutes.
Remove from heat and set aside.
Pat dry with paper towels and season ribs with kosher salt and black pepper.
Prepare a grill fire to about 400° using hickory or pecan makes for great smoke flavor.
Place ribs, meaty side up over indirect heat. Lower grill to 325°.
Cook with the grill closed for about 1 1/2 to 2 hours until internal temperature reaches 195° to 200° using a digital meat thermometer.
Remove ribs to a large sheet pan.
Brush with mustard glaze.
Return to the grill and continue to cook another 20 -3o minutes keeping the grill between 300° to 325°.
Remove from grill and rest about 10 to 15 minutes before slicing.
Serve with additional mustard sauce if desired.