In a medium bowl, combine the chicken pieces with all the marinade ingredients. Toss to coat evenly. Cover and refrigerate for at least 30 minutes, or up to 4 hours.

In a large skillet, heat the olive oil over medium heat. Add the chopped onion and cook until softened, about 5-7 minutes. Add the minced garlic and grated ginger and cook for another minute until fragrant.

Stir in the garam masala, turmeric, cumin, and cayenne pepper (if using). Cook for 30 seconds, stirring constantly, until fragrant.

Transfer the sautéed onion mixture to the slow cooker. Add the crushed tomatoes, coconut milk, and chicken broth. Stir to combine.

Add the marinated chicken pieces to the slow cooker. Add the cauliflower florets. Stir everything together to ensure the chicken and cauliflower are submerged in the sauce.

Season with salt and black pepper. Cover and cook on low for 4-6 hours, or on high for 2-3 hours, until the chicken is cooked through and tender and the cauliflower is tender-crisp.

Taste and adjust seasoning as needed. Stir in half of the chopped cilantro before serving.

Serve hot, garnished with the remaining fresh cilantro. This dish is delicious served with basmati rice or naan bread.