Melt one tablespoon butter in a large stockpot over medium-high heat. Add onion, garlic, ginger and sauté for 4-5 minutes, stirring occasionally. Add the garam masala, chili powder, coriander, cumin, turmeric and sauté for 2 minutes, stirring frequently. Add the diced tomatoes and water and stir to combine, using a wooden spoon to scrape up any browned bits on the bottom of the pan. Use an immersion blender (or see note below if you prefer to use a traditional blender) to purée the sauce until smooth. Continue cooking the sauce until it reaches a simmer. Stir the cauliflower into the sauce and simmer, uncovered, for 10 minutes, adjusting the heat as needed to maintain a low simmer. Stir the chicken into the sauce and continue to simmer, uncovered, for 10 more minutes or until the chicken and cauliflower are cooked through and tender. Stir in the spinach, heavy cream (or coconut milk), lemon juice and remaining two tablespoons butter until combined. Taste and season with salt (I used about 1 teaspoon), black pepper, and/or extra lemon juice if needed. Serve. Serve immediately over rice with naan, garnished with lots of fresh cilantro and extra lemon wedges. Enjoy!