Place the peach halves & yellow onion on a large plate or a small baking sheet. Lightly drizzle with avocado oil & toss to coat. Place the peaches & onion directly on the grill grates. Grill 4-5 minutes per side, or until softened & nicely charred. Transfer the grilled peaches & onion back to a plate or small baking sheet. Once cool enough to handle, carefully peel back the peach skin & discard.
Place grilled peaches & onion in a blender with the chipotle peppers, brown sugar, apple cider vinegar, ketchup, & Worcestershire sauce. Season with S&P as desired. Blend until smooth. Taste & adjust seasoning as needed or desired – add a little more chipotle for smoky heat, brown sugar for sweetness, vinegar for tanginess, Worcestershire for richness, etc. Transfer to a jar or bowl & set aside for grilling.
Place chicken on a plate or small baking sheet. Season with chili powder, garlic powder, paprika, & S&P as desired. Use your hands to rub the spices all over the surface of the chicken. Place the chicken on the preheated grill. Close the lid & grill 4-5 minutes per side. About 2 minutes before everything is finished grilling, brush the prepared chipotle peach BBQ over top, flipping such that the glazed sides come into contact with the grill grates. Cook 1 minute to caramelize the sauce. Repeat, brushing & grilling the second side. Transfer the grilled chicken to a plate or small baking sheet.