IndianFoodForever provides a good basic chaat recipe for ragda patties. I'd add more tomato and make them very finely diced.
For the Patties (can be done ahead):
3 large Potatoes
1/2 tsp ginger, grated
1/2 tbsp Garam Masala
1/4 tbsp Turmeric
2 Tbsp Bread crumbs
Peel, quarter, and boil the potatoes until soft, then mash them or pass them through a ricer.
Add spices, mix and knead well into a dough.
Add the bread crumbs and knead well.