Toast some peanuts in a skillet over medium heat until golden and beginning to brown in spots. Add a little oil and some chili flakes and toss to combine. Remove from the heat and transfer to a plate to cool.
For the dabeli:
Put the potato into a bowl. Sprinkle with some salt, the amchur, red chili powder, coriander, dabeli masala and turmeric. Heat a little bit of oil in a medium skillet over medium heat. Add the cumin seeds and toast until fragrant. Add the potatoes and spices to the pan and mash with a potato masher to mix well. Remove from the heat.
Butter the cut sides of the rolls and toast both sides in a clean pan until golden brown. Spread the garlic chutney on 1 cut side of the roll and the tamarind chutney on the other. Spoon some of the potato mixture on the bottom half of the roll and garnish with onions and peanuts. Close up the sandwich and slice in half.