In a medium saucepan, drizzle the oil and place over medium-high heat. Add the red onions and stir, reducing the heat when they start to sizzle. Cook, stirring, until the onions are soft, about 5 minutes. Add the ginger and stir for a minute, then increase the heat to medium-high, add the mustard seeds and stir for a minute longer. Add the peaches, brown sugar, lemon juice, salt, turmeric, cinnamon and cayenne and stir until the mixture starts to bubble.