Preheat the oven to 375 degree.
Whisk together the vinegar, mustard, honey and thyme in a medium bowl and season with salt and pepper. Add the olive oil and whisk until emulsified.
In a large salted pot of water, boil the potatoes until cooked through but not soft, 10 to 15 minutes. Drain and cool off with cold water to be handle them.
Cut them in half, place them in a bowl with olive oil and sprinkle generously with salt. Place them in a cooking sheet and bake for another 10 minutes in the hot oven
Arrange the watercress on a platter, top with the potatoes and drizzle vinaigrette over the top and serve immediately.
Season with salt, pepper and fresh chopped thyme.
Cook on medium high heat for 4 minutes on each side. Each side should be a golden crust. Place in the hot oven for 5 minutes.
Remove from oven, give it a nice last searing on the stove. Turn heat off and cover with aluminum foil.
Transfer to a platter and spoon vinaigrette over each serving