Heat oil in a large pot or dutch oven over medium heat, then add onions and cook until soft and golden, about 15 minutes (this is an important flavour-building step, don't rush it). Push the onions to one side and add the cumin seeds to the other along with another glug of oil. When they're a shade darker and fragrant, add the jalapeno, ginger, and garlic and cook, stirring everything together for a minute. Add remaining spices and stir to coat, then add tomatoes. Cook this mixture for a few minutes, until you see oil shimmering on the top.