In a slow cooker, heat the oil using the saute setting. Add the onion and cook, stirring occasionally, until tender, about 8 minutes. Add the garlic, ginger, coriander, garam masala, cumin, and turmeric and cook until fragrant, about 1 minute. Add the tomato paste and cook until darkened in color, about 1 minute. Add the drained tomatoes and 1/2 teaspoon of salt and bring to a simmer, scraping up the browned bits from the bottom of the pan.