For the dressing, whisk the mustard, vinegar and garlic together in a small bowl, then very gradually whisk in the oil to make a thick, emulsified dressing. Season to taste with salt and pepper and set to one side.
Grill or fry the sausages for 8 minutes, turning them over now and then, until nicely browned all over.
Meanwhile, overlap the tomato slices on a large, oval platter and sprinkle with a few sea salt flakes and ground black pepper. Scatter over the shallots and the capers.
Lift the sausages onto a board and slice each one on the diagonal into 4 pieces. Whisk 2 tablespoons of warm water into the dressing to loosen it a little, then drizzle over the salad.
Arrange the sausages randomly over the top of the tomatoes, sprinkle with the chopped parsley and serve while the sausages are still hot.