1. In a saucepan heat olive oil on medium heat and add mustard seeds and Asafoetida. Allow seeds to sputter and crackle for a few seconds and then add in onion, garlic, ginger, and chillies. Saute until softened and fragrant.
2. Next add in the tomatoes, chilli powder, coriander, cumin, vinegar, sugar, and salt to taste. Bring to a simmer and cook on medium-low heat for 30 minutes, stirring occasionally until jammy and thickened. Remove from heat, and allow to cool.
3. To assemble the sandwich, butter 2 pieces of bread on one side. Spread chutney on the other side of each slice and sprinkle chopped coriander over. Top one slice with a generous handful of both types of cheese. Place the other slice of bread on top, with the buttered side facing upwards.
4. Heat a grill or pan on medium heat. Place sandwich on pan, and cook for 3-4 minutes on each side until deeply golden and crisp, and gooey in the middle.
5. Slice in half and enjoy piping hot!