Easy Butter Chicken (CrockPot)

4 skinless, boneless chicken breasts
1 onion, halved and thinly sliced
1 Tbsp. grated fresh ginger
4 garlic cloves, crushed
1 Tbsp. chili powder
2 tsp. curry paste or powder
a pinch of cinnamon
1 28 oz. can diced tomatoes, undrained
2 Tbsp. tomato paste (optional)
1-2 tsp. garam masala 
1 cup plain greek yogurt
1/2 cup(ish) half & half or whipping cream
salt and pepper to taste
steamed brown basmati rice, for serving

Method: Place everything but the yogurt, cream, salt and pepper into the CrockPot, cover and cook on low for 4-6 hours. Uncover, stir in the yogurt and cream, season to taste with salt and pepper, and serve hot over rice mixed with green peas.