For the dressing:
1) Whisk together the mustards, honey, vinegar and salt and pepper in a large bowl. Slowly whisk in the olive oil until emulsified and stir in the fresh coriander. Let sit at room temperature until ready to use.
For the grilled fingerling potato salad with lobster and watercress:
1) Heat griddle to medium. Brush potatoes with rapeseed oil and season with salt and pepper, to taste.
2) Griddle until lightly golden brown on both sides and just cooked through, about 2 minutes per side. Add the warm potatoes to the dressing, and toss to coat. Add the lobster and the watercress, season with salt and pepper and toss to combine. Transfer to a platter and serve.