Heat the oil in a wide pan over medium heat. Add the mustard seeds and allow them to splutter. Add the cumin seeds and let them sizzle for a few seconds.
Add the grated garlic and chopped green chilli. Sauté briefly until aromatic, taking care not to brown the garlic.
Add the finely chopped onion and sauté until it turns translucent.
Add the chopped tomato and cook until it becomes soft and slightly mushy.
Add the salt, Kashmiri chilli powder, garam masala, coriander powder, and turmeric powder. Mix well and cook for about 30 seconds.
Add approximately ¼ cup water and stir well. Cook until the raw smell of the spices disappears and the mixture becomes almost dry.
Increase the heat to high. Add the grated carrot, shredded cabbage, and chopped bell pepper one after another, stirring quickly after each addition.
Stir-fry the vegetables over high heat until they are lightly cooked but still retain a little bite. Do not overcook them or allow the mixture to become watery.
Add the tomato ketchup and mix until it evenly coats the vegetables.
Add the chopped cilantro, mix well, and turn off the heat. Let the masala filling cool slightly.
Heat a skillet over low-medium heat. Spread softened butter on one side of each slice of bread. Place the bread slice on the skillet with its buttered side facing down.
Spread a generous layer of the prepared vegetable masala over the bread, leaving a small border around the edges. Cover it with another bread slice, keeping the buttered side facing upward.
Toast the sandwich until the bottom becomes crisp and golden brown. Carefully flip it and toast the other side, pressing gently with a spatula.
Remove the sandwich from the skillet, slice it into halves or triangles, and serve warm.