In a medium saucepan, warm 2 tablespoons of olive oil over medium heat. Sauté 1 finely chopped sweet onion until tender and translucent, about 4-6 minutes. Add 3 minced garlic cloves and stir for an additional minute. Mix in 2 cups of chopped rhubarb, 1 cup of ketchup, 1/2 cup of water, 1/2 cup of apricot preserves, 1/4 cup of cider vinegar, 2 tablespoons of molasses, 2 tablespoons of honey Dijon mustard, and 1 finely chopped chipotle pepper. Sprinkle in 1 teaspoon of barbecue seasoning along with salt and pepper to taste. Bring to a boil, then reduce heat to low and simmer for about 8-10 minutes. Use an immersion blender to puree the sauce until smooth, reserving 2 cups for serving.

Mix the remaining barbecue seasoning with a generous pinch of salt and pepper in a large bowl. Toss chicken drumsticks in the seasoning mixture until evenly coated. Allow resting for about 10 minutes.

Preheat the grill to medium heat, aiming for around 350°F to 375°F. Set up an area for indirect grilling.

Place seasoned chicken drumsticks over indirect heat. Grill for about 15 minutes. Turn the drumsticks over and move them to direct heat, continuing grilling for another 15-20 minutes, or until the internal temperature reaches 170°F - 175°F.

Remove chicken from the grill and let it rest for a few minutes. Serve with reserved sauce on the side.