Boil potatoes in lightly salted water until they are soft. Drain and mash roughly, leaving a few larger chunks.
Meanwhile, heat oil in a frypan over a medium to high heat. Add onion and garlic and saute for 3-4 minutes until soft and translucent.
Add whole spices: cumin seeds, fennel seeds, black mustard seeds and chilli flakes and cook for one minute until fragrant.
Follow this with the ground spices: ground cumin, ground coriander, asafoetida, garam masala, turmeric and salt. Saute for a further one minute.
Add defrosted peas and crumbled potato. Turn heat to low and fold spices and oil into vegetables. Remove from heat and set aside to cool for 30 minutes.
Cool sandwich by heating up jaffle iron. Spray lightly with oil spray if required. Place one slice of bread down and fill with a quarter of the potato filling. Top with another slice of bread then shut jaffle iron. Cook until both sides of bread are golden and crispy.
To make the coriander yoghurt: place all ingredients into a food processor and blitz until coriander is fine. Serve on the side of jaffles.