In the bowl of a large slow cooker, stir together the diced onion, tomato paste, chicken broth, curry paste, cardamom, and garam masala until combined.
Nestle the chicken breasts into the mixture so they are covered with the sauce.
Place the cover on the slow cooker and cook on LOW for 6 hours.
During the last two hours of cooking, stir in the sliced mushrooms.
Turn the slow cooker off. Remove the chicken to a large plate and shred with a fork.
Stir the coconut milk into the tikka masala sauce until combined, then stir the shredded chicken into the slow cooker until it's completely covered with sauce.
Serve immediately over cooked rice with, if desired, a sprinkling of fresh cilantro, pistachios, raisins, and tzatziki sauce.