Prepare the bell pepper by taking out the stem and the seeds. Chop into small pieces. Then chop red chillies and onions into slightly smaller pieces.
Place the bell pepper, red chillies, onions, garlic, and 1 cup of water into a blender. Blend then transfer this into a pan and bring to a simmer at medium heat.
Pour in ½ cup water, salt, brown sugar, pepper, and vinegar. Mix well. Add in the cornstarch slurry. Mix until it thickens.
Once it feels as thick as you like, take this out and let it cool to room temperature before using.