Cooking Rack of Lamb on a gas grill

I half agree with Alexa. The melting fat from the lamb is dripping into the flame directly below the lamb and flaring up. This causes the cooking meat to catch fire and burn.

What I do to prevent these flareups is to preheat the grill with all burners on for 15 minutes. This gets the grill irons good and hot.

Now most gas grills have at least two sets of burners which offers some useful flexibility. Once the grill is preheated, I turn off one side while leaving the other side on high. Finally I cook the lamb on the side with the burner off and the top closed to keep the heat in. Thus the grill irons sear the meat on the outside while the indirect heat safely cooks the meat to your desired internal temperature.

See Cooking Temperatures for Lamb for more.