Heat oil in a large saucepan. Add onions, tomatoes and chili pepper. Saute until onion is translucent, about 3-4 minutes. Add garlic, ginger, chili powder, coconut milk, turmeric, and broth. Simmer, uncovered, until sauce thickens, about 15-20 minutes. If sauce becomes too thick, thin with some more broth or water.