Heat oil in a saucepan and add the onions. Saute until they turn golden-brown, then add in the ginger-garlic paste. Stir-fry for a minutes, then add in the tomatoes and tomato paste.
Cook the tomatoes, stirring frequently, until they become very pulpy and darken slightly. Add the powdered spices — the coriander powder, cumin powder, garam masala, paprika and turmeric. Mix well and cook for another couple of minutes.
Add 2 cups of vegetable stock to the saucepan and mix well. Chop the "chicken" tikka pieces into smaller, bite-sized bits and add to the sauce. Bring everything to a boil.
Add the cashew paste and salt to taste. Add the green peppers and shallots. If the sauce seems too thick at this point, add some vegetable stock. The sauce will thicken as it cooks in the oven, so don't make it too thick at this point. Once you have the desired consistency, turn off the heat.