Prepare the chutney: Add the coriander, spinach, fresh ginger, garlic, fresh chillies, raw peanuts, lemon juice, and water to a small blender and blend until smooth.
Make the masala onions: Add the sunflower oil, mustard seeds and cumin seeds to a small pan over medium heat. Once the seeds begin to crackle, lower the heat and add the sliced red onion. Stir to coat the onions in the oil, then add the spices: ground coriander, ground cumin, turmeric, and chilli powder. Stir to combine and cook for 5 minutes until softened.
Assemble the sandwich: Generously spread the margarine over 4 slices of bread, ensuring to go all the way to the edges. Add one slice of bread, butter side down, to a frying pan over medium-low heat. Add a layer of cheese, then half the masala onions, and some tomato slices. Top with another layer of cheese and the final slice of bread, butter side up. Repeat for the second sandwich.
Cook the grilled cheese: Cook the masala grilled cheese for 3-5 minutes on each side until golden and crisp. Ensure the cheese has melted before serving the sandwich hot alongside the chutney. Enjoy!