In a 10-inch skillet
, heat the oil
over medium-high heat until hot. Add the mustard seeds and cover. The seeds will start popping; cook until the popping subsides, about 1 minute. Uncover, reduce the heat to medium, and add the butter. As soon as it melts, add the onion, ginger, garlic, cumin seeds, and coriander and cook, stirring, until the onion is soft, about 3 minutes. Add the tomato, chile, and 1 teaspoon salt and cook, stirring, until the tomato begins to soften and the chile is fragrant, about 2 minutes.