In a heavy bottomed saucepan heat oil on medium.
Add the mustard seeds when the oil is hot enough (if using mustard oil don't over heat it)
When they start crackling and spitting, add the tomatoes. (use a lid to cover if it starts spitting )
Add the salt and sugar.
Reduce heat to low and simmer,
Keep it partially covered for 25 minutes.
Stir it every 5 minutes to ensure the chutney doesn’t stick.
Stir through the garam masala.
Cook for 2 more minutes and remove from heat.
Cool completely and store in sterilised glass jars in the fridge for a week.