First prepare all the chutneys and put aside (or they can be store-bought).
For mint chutney, grind together 1 cup mint leaves, 1 cup coriander leaves, 1 green chilli, ½" ginger, 1 tsp cumin powder, 1 Tbsp lemon juice and a pinch of black salt or salt. This chutney can be stored in an air-tight container in refrigerator.
For tamarind chutney, it needs ½ cup seedless tamarind soaked in 1 cup warm water. In a pan, heat 1 tsp oil and add 1 tsp cumin seeds and let them splutter. Add ½ tsp ginger powder, a pinch of asafoetida and ¼ tsp red chilli powder. Add the squeezed out tamarind juice and cook for 3 minutes. Add 8 Tbsp jaggery or according to the taste and cook for 5 minutes. This mixture would thicken. Let cool and store in an air tight container and store in refrigerator.
These chutneys can prepared a day ahead and kept ready.
Beat the yogurt until smooth.
On a plate, arrange the puris or golgappas - around 6 or as many you want to serve. Break the puris from the center to create space for the potatoes and chutneys.
Stuff the puris with potatoes, onions and tomatoes. Sprinkle chaat masla, cumin powder, black salt and red chilli powder.
Pour the chutneys one by one all over the stuffed puris.
Add the beaten yogurt little by little on each puri.
Add sev and chopped coriander on top of all puris.
Serve dahi sev puri as soon as you make it as the puris become soggy if you serve even after a few minutes. Assemble the plate just before serving.