Position a rack in the center of the oven and heat the oven to 350 degrees F. Halve the squash lengthwise and scoop out the seeds with a sturdy spoon. Set the squash halves cut side down on a heavy-duty rimmed baking sheet, and bake until strands of flesh separate easily when raked with a fork, 50 minutes to 1 hour. Taste a few strands-they should be tender. If not, continue to bake. Set the squash halves aside until cool enough to handle. Use a fork to rake the cooked squash flesh into strands.

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.

Increase the heat to medium high and add the squash to the skillet. Continue to cook, tossing with tongs, until heated through, 1 to 3 minutes. Toss in the cilantro, season to taste with more salt, and serve.

Make Ahead Tip: You can bake the squash in advance and then finish the dish right before serving.