Heat oil in a large saucepan over medium heat. Add onion and cook, stirring frequently, until soft and starting to brown, 3 to 4 minutes. Stir in ginger, garlic, garam masala, cumin, coriander and ¼ teaspoon salt; cook, stirring, for 1 minute. Add tomato paste and cook, stirring, for 30 seconds. Stir in coconut milk and bring to a simmer. Cook, stirring occasionally, until slightly thickened, about 5 minutes. Remove from heat.