Heat oil in a kadai or wok over medium heat. Add chopped onions and ginger-garlic paste. Sauté until onions turn translucent.
Add turmeric powder, red chilli powder, coriander powder and garam masala. Sauté for a minute.
Add finely chopped toamtoes and cook for a few minutes until the get cooked well and well mashed.
Add tomato ketchup. Mix again. Add little water and bring it to a rolling boil. Add salt to taste.
In a bowl, add cornflour and little water. Make a thick slurry without having any lumps. Add the cornflour slurry to the gravy and mix well.
Adjust the consistency of the gravy by adding water, because once the mushroom cabbage fritters are added, the gravy will completely thicken.
Add the fried mushroom fritters to the boiling gravy.
Cook on high heat, stirring gently, until the gravy thickens and coats the fritters. The final consistency should be semi-dry, with the fritters well-coated in the spicy gravy.
Transfer the roadside kalan to a serving plate. Garnish generously with finely chopped onions, and fresh coriander leaves.
Serve immediately with lemon wedges on the side.