Pre-heat oven to 180°C / 160°C fan.
In a small bowl, mix together passata, water, BBQ sauce, sriracha and onion salt.
Arrange chicken thighs in a baking dish then pour sauce over. Cover dish tightly with tinfoil then bake for 1 hour.
After the hour, remove tinfoil from dish, increase oven to 200°C / 180°C fan and bake for further 20 -30 minutes to thicken the sauce.
While chicken is cooking, prepare the slaw. In a small bowl, mix together the mayonnaise, lemon juice, vinegar, mustard and salt.
Toss together cabbage, carrot and onion. Add the mayonnaise dressing and toss well to combine. Refrigerate until needed.
Remove chicken from dish and place in a shallow dish. Use two forks to pull apart the chicken. Spoon over some of the sauce and mix well. Add more sauce if needed.
To serve, spoon pulled chicken onto the bottom half of the brioche buns. Spoon over more of the cooking sauce, optional. Top with a mound of tangy slaw, then pop the tops of the buns on. Serve immediately.
Sliced dill pickles is also a delicious addition.