Mix the garlic, half the ginger, mustard, chilli powder, stevia and a pinch of salt and pepper.
Spread over the salmon fillets and set aside to marinate in the fridge for 30 minutes.
Heat the oil in a frying pan over medium heat. Fry the onion for 5 minutes, then reduce heat to low and fry the onion for 20 minutes, stirring frequently, until caramerlised.
Add the vinegar and stir through for 5 minutes. Season to taste with salt, pepper, and stevia (if it is not sweet enough) and set aside.
Heat a non-stick frying pan over medium-high heat. Fry the salmon, skin side down, for 3 minutes. Turn over and fry for another 2 minutes or until the salmon is cooked through then remove to a serving dish.
Add the onion to the pan the salmon was cooked in and stir in the leftover ginger and water. Cook for 2 minutes over medium heat.
Pour over the salmon fillets and garnish with the basil leaves.