Begin by preparing the batter for the paneer: in a medium-size mixing bowl, stir together the flour, cornstarch, salt and water to make a batter.
Add the chopped paneer cubes to the batter and toss until they are lightly coated.
Heat up the 1/4 cup of sunflower oil in a large frying pan or wok over medium heat. Test the oil is hot by adding a tiny amount of the batter to the oil - if it sizzles, the oil is ready.
Use tongs to carefully add the paneer cubes into the hot oil one by one. Make sure that you drain as much batter from the paneer before adding it to the oil - we just want a light coating of batter on the paneer.
Fry the paneer cubes for 5-7 minutes, turning occasionally, until they are golden on all sides. Transfer the paneer to a plate lined with a paper towel so the excess oil can drain, and set it aside for now.
Drain most of the oil from your frying pan leaving around 1/2 a tablespoon behind. Add the chopped peppers and red onion to the pan and cook over medium heat until they are just slightly charred - try to retain some of the colour and crunch in the peppers. Transfer the peppers and onions to a bowl and set aside.
Now we can prepare the gravy: add the sesame oil, chopped ginger, chopped garlic and chopped spring onions to the frying pan. Cook until slightly fragrant, then add the tomatoes and chopped green chillies.
Once the tomatoes are slightly mushy, add the spices: 1/4 tsp ground cumin, 1/4 tsp ground coriander, 1/4 tsp chilli powder, and 1/3 tsp turmeric. Cook out the masala until it begins to smell good, stirring occasionally.
Add the tomato ketchup, soy sauce, chilli sauce and water to the pan. Stir everything together to make the gravy. If you prefer a thinner gravy, add some extra water at this stage.
Add the fried peppers and onions and crispy paneer cubes back into the pan with the gravy. Stir everything together to fully coat the paneer.
Ensure the chilli paneer is heated through before serving. Garnish with the reserved chopped spring onion and enjoy immediately whilst the paneer is still crispy.