To a blender, add the tomatoes, roughly chopped onions, salt, dry red chillies and blend to almost smooth blend.
Heat a deep pot with a lid or a kadai, and add oil to it and once hot, add the mustard seeds and allow it to crackle fully, add the cumin and fennel seeds and allow it to sizzle for a couple of seconds.
Quickly add the blended onion and tomato mixture, turmeric powder, fennel seed powder. Once you add it will bubble so keep it covered and cook for 5 - 10 mins on medium flame.
Open the lid and give it a stir and cook for 5 more minutes. The chutney will be brighter in red and not very thin not too thick.
Serve it hot over idles or dosa and enjoy !!!