To prepare the filling, heat 1 tbsp of olive oil in a pan. Add in onions and cook until translucent. Add in ginger garlic paste and saute for a minute.
Next, add the mushrooms with a teaspoon of salt. Mix well. Cover it with a lid and cook until the juices dry up.
Add in the shredded chicken and cut basil leaves. Mix well and keep aside.
In a bowl, prepare the custard. Whisk together eggs, cream, mustard paste, Worcestershire sauce, all-purpose flour, and cornflour. Keep aside.
In the meantime, preheat the oven to 180°C or 350°F.
Grease the baking dish. Just before assembling, add baking powder to the custard and mix well.
To assemble, add the filling to the baking dish and gently even it out. Top it with a cup of grated cheese.
Pour in the custard. Make sure it seeps in well.
Garnish with halved cherry tomatoes and a few basil leaves.
Bake in a preheated oven at 180°C or 350°F for 20-30 minutes or until done. Check with a skewer or a knife. It should come out clean. The Cheesy Chicken Casserole is ready to serve.