Wash the squash. Cut off both ends and discard. Slice into thin circles, about 1/4-inch (6 mm) thick.
Pour the oil into a medium saucepan and place over medium heat. When the oil is heated, add the onion. Sauté until the onion is golden brown, stirring frequently, about 5 minutes. Reduce the heat to medium-low and add the tomato. Cover the saucepan and cook until the tomato becomes completely soft and mashed and is combined with the onion to form a coarse paste, stirring every minute or so and lightly mashing the tomato, about 5 minutes.
Add the cumin seeds, turmeric, red pepper, salt and black pepper. Stir to combine. Cook uncovered for 3 minutes, stirring frequently.
Add the squash. Stir to combine. Cook for 2 minutes, stirring frequently.
Add the water. Bring to a rolling boil over high heat. Stir and reduce the heat to medium-low. Simmer until the squash is tender, stirring occasionally, about 10 minutes. Some of the water will cook off, but the dish will still have a curry base. Enjoy now or let cool to room temperature and refrigerate or freeze for later!