For the vinaigrette, place the mustard into a bowl and whisk in the vinegar, water and olive oil. Season with salt and white pepper. Use a hand blender to mix thoroughly until thickened and emulsified. Add the chopped tarragon.
For the salad, place the mixed leaves, herbs and shallots into a bowl and gently toss. Dress the salad lightly with the vinaigrette just before cooking the scallops.
For the scallops, heat a little olive oil in a non-stick pan over a medium heat. Season the scallops with salt and freshly ground black pepper before placing into the pan sliced-side down and cook until golden-brown, about 1-2 minutes. Turn the scallops over and reduce the heat. Cook for a further 1-3 minutes, depending on their size, or until just cooked. Take care not to overcook the scallops.
Remove the scallops from the pan and place onto a tray lined with kitchen paper. Squeeze over the lemon juice.
To serve, place a little of the dressed salad onto four plates. Place the scallops on and around the salad. Finish by drizzling some of the grain mustard vinaigrette around the plate.