Mix all dry rub ingredients in a small bowl.
Pat chicken thighs completely dry. Season with dry rub on both sides. Press firmly to adhere.
Heat oil in a large heavy pot over medium-high heat. Sear chicken 3 to 4 minutes per side until golden. Transfer to a plate.
Reduce heat to medium. Cook diced onion in the same pot for 3 to 4 minutes until softened.
Add all BBQ sauce ingredients to the pot. Stir until smooth. Taste and adjust balance of sweet, tangy, and smoky.
Return seared chicken to the pot. Nestle into the sauce until well coated.
Bring to a gentle simmer. Reduce heat to low. Cover and cook for 20 to 25 minutes until chicken is completely tender.
Verify internal temperature reaches 74°C (165°F).
Remove chicken with tongs. Rest for 5 minutes.
Increase heat to medium-high. Simmer sauce uncovered for 5 to 8 minutes until thickened and glossy.
Shred chicken with two forks into long rough strands.
Return shredded chicken to thickened sauce. Toss until every strand is generously coated. Taste and adjust final seasoning.
Toast buns cut-side down until golden.
Pile pulled chicken onto toasted buns. Top with coleslaw, pickles, extra BBQ sauce, and spring onions.
Serve immediately.