Heat the oil in a large skillet over medium-high heat. Add the onion, and season with salt and pepper. Cook, stirring occasionally, until the onion starts to soften, about 3 minutes. Stir in the garlic, ginger, curry powder and cumin. Cook, stirring, until fragrant, about 1 minute longer. (Alternatively, combine everything, including the spices, in a heatproof bowl and microwave on high, stirring occasionally, until softened, about 4-5 minutes.) Scrape the mixture into a 4- to 7-quart slow cooker.