Heat the oil in a medium saucepan of medium heat.  Add the onions and sautè for 3-4 minutes, until softened. Add the garlic and cook for another minute. 

Add in all of the remaining ingredients and stir until combined.  When the sauce starts bubbling, lower the heat to medium-low and simmer for 20-25 minutes, stirring frequently, until the rhubarb is tender. 

Remove from the heat and transfer to a blender or food processor and blend until smooth.  Be careful if the sauce is still hot as it can create pressure with the lid on.  I just cover the top of my blender with a towel while I blend the hot sauce.  The sauce will be thicken as it cools, but may be thinned with water to reach your desired consistency.

Store in the refrigerator and use to baste chicken,pork chops or tenderloin, ribs, or other meat on the grill, or use it with pulled pork or in any recipe that calls for BBQ sauce.

Heat up the grill to medium-high with a closed lid until it reaches 400 degrees F.  Season the chicken with salt.

When the grill is hot, turn off one burner and brush that side of the grill with vegetable oil.  Set the chicken pieces on that side of the grill (indirect heat) skin side down and shut the lid. 

Cook for  7-10 minutes on the first side before flipping, closing the lid again, and cooking for another 7-10 minutes. 

Flip the chicken again and move to the area of the grill that is directly over the flames (the direct heat area). Grill just until the skin is well browned and crisp, about 3-5 minutes.  

Flip the chicken and brush with rhubarb bbq sauce, then finish cooking until the chicken reaches165 degrees for breasts or 180 degrees for legs and thighs when tested with an instant read meat thermometer.

Serve with extra rhubarb bbq sauce on the side.