Heat the canola oil in a wok or wide skillet over medium-high heat, and saute onion for 3 minutes, until just softened. Add 3 tablespoons coconut milk to pan, and bring to a boil. Stir in curry paste. Mix in remaining coconut milk, broth, sugar and fish sauce, and bring to a boil. Reduce heat, and simmer vigorously for 5 minutes.