Halve the chicken breasts horizontally (to create two thin fillets) or use a meat mallet to flatten the chicken out slightly. Place in a Ziploc bag.
To make the marinade, whisk together oil, lemon juice, Garam Masala, Turmeric, garlic powder, salt and pepper. Pour the marinade over the chicken and allow to marinade for a few minutes or up to 24 hours in the fridge.
Heat an outdoor grill or grill pan over high heat then cook the chicken for 2-3 minutes per side until golden and cooked through. Remove from the heat and allow to rest for a few minutes.
Thinly slice the cabbage, fennel and red onion and place in a bowl. Add coriander/cilanto. Whisk together mayonnaise, yogurt/sour cream and lemon juice and season to taste with salt and pepper.
Spread some mayo on slices of bread then top with the grilled chicken. Add a generous spoonful of slaw and sandwich with another slice of bread. Serve immediately.