Arrange the tomato slices in a circle on a serving platter.
Scatter the red onion over the top.
Place the shallot, garlic, mustard and white wine vinegar into a bowl and whisk well to combine. Whisk in the rapeseed oil, then add the water (whisking continuously) to emulsify the dressing. Season to taste with salt and freshly ground black pepper.
Spoon the dressing over the tomatoes then set aside for at least one hour to marinate.
When ready to serve, scatter over the parsley and basil leaves.