Sprinkle the salmon with salt and pepper, then lightly dredge in cornflour. Let sit for 10 minutes.
Heat a large frying pan with 1 tablespoon oil over high heat. Sear the salmon on both sides, then turn the heat down to medium and cook until 90% done, about 5-6 minutes total, transfer to a serving plate.
Heat the same pan with 1 tablespoon oil over high heat. Stir-fry the garlic until aromatic, 2-3 minutes. Add the onion and salt, cook until soft, 3-4 minutes.
Stir in the tomato, cook for 2 minutes. Stir in the tomato ketchup, water, passata, sweetcorn and season to taste. Bring the sauce to the boil, turn off the heat, sprinkle in the Cheddar and pour over the salmon.