In a large sauté pan with a lid or a Dutch oven, heat the oil over medium-high heat. Stir in the ginger and serrano and cook, stirring, until fragrant, about 30 seconds. Stir in the garlic and the spice mixture and stir until fragrant, about 10 seconds. Add the cauliflower and potatoes and stir to coat everything with the spices. Pour in 3⁄4 cup water while stirring. Bring to a boil, then reduce the heat to medium-low, cover, and cook until the vegetables are tender, about 25 minutes, checking the pan once or twice to be sure the liquid is bubbling, not steaming. (If the pan becomes dry, add 1⁄4 cup water.)