Warm oil over medium-high heat in a medium saucepan, until nearly smoking.
Keep your lid handy. Add the mustard seeds and the cumin seeds; they should start enthusiastically popping upon contact with the oil, so cover the pot until the spluttering subsides.
Add the onions, garlic and ginger. Saute until softened and just starting to brown, about 10 minutes.
Add turmeric, garam masala and paprika. Stir and cook about 30 seconds. Then add the vinegar (standing back so you don’t inhale the fumes!), and cook until vinegar has reduced by about half.
Add tomatoes, molasses, salt and lots of freshly ground black pepper. Stir, bring to a boil, then cook at a generous simmer about 10 minutes until thickened. You can puree it if you like. Set aside to cool, then jar. Store in the refrigerator, ready for your next burger or sandwich or grilled meat or or or...!