Whisk mustard, vinegar, water, and olive oil in a bowl and season with salt and white pepper.
Emulsify the mixture with a hand blender until thickened and stir in the chopped tarragon.
Toss the mixed leaves, herbs, and shallots together in a salad bowl.
Lightly dress the salad with the vinaigrette immediately before cooking the scallops.
Heat olive oil in a non-stick pan over medium heat and season the scallops.
Sear scallops sliced-side down for 1-2 minutes until golden-brown.
Turn scallops, reduce heat, and cook for 1-3 minutes until just cooked through.
Drain scallops on kitchen paper and squeeze lemon juice over them.
Arrange the dressed salad on four plates and place scallops on top.
Drizzle the remaining grain mustard vinaigrette around the plates to finish.