Heat the vegetable oil in a dutch oven or other large pot.
Cook the onions over medium heat until softened.
Add garlic and ginger, continue to cook until fragrant and softened.
Remove from heat, add Curry powder, cumin, and coriander.
Pour 1/2 cup water into a blender. Add cooked vegetables and blend on high until very, very smooth. Add additional water if necessary.
Pour sauce back into pot and stir in tomato paste and Chilis, if using.
Cover and cook for at least 30 minutes, but no more than an hour. -Note- Sauce will bubble and gloop violently all over your range if you do not cover.
Add chicken pieces, cook until tender and 160 – 170 degrees F.
Remove from heat, stir in cilantro and yogurt and serve over rice.