While the vegetables are roasting, make the curry sauce. To do so, add about 1/3 cup of the coconut milk to a saucepan, along with the onion, ginger, and garlic. Saute, stirring occasionally, until the onion is soft and translucent, and the coconut milk has been absorbed, about 10 minutes. Add the spices and saute an additional 3 minutes. Add the remaining coconut milk and diced tomatoes. Bring to a full boil, then reduce the heat. Allow the sauce to continue cooking at a controlled boil, stirring occasionally, until thick, about 15 minutes.