Roughly chop the tomatoes and onion, grate the ginger, and mince the garlic. Add the olive oil to a pot over medium heat. Add the onions, ginger, garlic, and crushed red pepper.
Step 2
Cook for 5 minutes, stirring often. Add the tomatoes and cook for 5 more minutes. Stir in the vinegar, mustard seeds, and sugar. Drop the heat to low and simmer for 10 minutes, or until the tomatoes have broken down and the mixture has thickened.
Step 3
Season with salt and pepper to taste. Pour the relish into a jar and let it cool completely. Transfer to the fridge to store.