Heat oil on medium/high in a medium sized pot.
Add chilies and black mustard seeds, let the seeds sputter.
Add hing powder and salt, lowering heat slightly.
Add diced onions and cook until translucent.
Add tomatoes and tomato paste and cook down until mixture thickens, or about 10 mins.
Using a hand blender, blend for 2-3 minutes. If using a food processor or blender, pulse 3-4 times until desired consistency. The chutney should be semi-chunky with pieces of onion and tomatoes still visible. Serve at room temperature. Keep in an airtight container in the fridge for up to 1 week.