My sister in law is an incredible chef and used to make lamb curry for us all the time. Once we talked about using beef and making it in the crockpot, and it worked out perfectly!!
We use the tzatziki sauce to calm some of the heat, I as often add more curry paste than is called for in the recipe (my husband loves spicy foods). You can also purchase a jar of mango chutney and serve with the meal; that's awfully tasty as well. Although many tzatziki sauce recipes call for mint, I don't add it to mine. However, I will often add a bit of greek seasoning mix to get a hint of mint flavor.
Place meat in slow cooker, add onions. Combine diced tomatoes, curry paste, and water in a bowl, pour over meat and onions. Cover and cook on low 8 hours or high for 4 hours.
Shred cucumber, pat dry using paper towels. Mix together yogurt, cucumber, and lemon juice. Mix spinach leaves into crockpot, stir well. Serve curry with rice and tzatziki sauce.