Preparation Combine vinegar, shallot, and mustard in medium bowl. Gradually whisk in oil. Mix in cheese. Season vinaigrette to taste with salt and pepper. Cook potatoes in large pot of boiling salted water until almost tender, about 16 minutes. Drain; cool completely. Cut potatoes into 1/4- to 1/2-inch-thick rounds. Brush rounds on both sides with oil; sprinkle with salt and pepper. Prepare barbecue (medium-high heat). Arrange watercress on platter. Grill potatoes until golden and cooked through, about 3 minutes per side. Mound warm potatoes atop watercress; sprinkle with green onions. Spoon vinaigrette over potatoes.
Preparation Combine vinegar, shallot, and mustard in medium bowl. Gradually whisk in oil. Mix in cheese. Season vinaigrette to taste with salt and pepper. Cook potatoes in large pot of boiling salted water until almost tender, about 16 minutes. Drain; cool completely. Cut potatoes into 1/4- to 1/2-inch-thick rounds. Brush rounds on both sides with oil; sprinkle with salt and pepper. Prepare barbecue (medium-high heat). Arrange watercress on platter. Grill potatoes until golden and cooked through, about 3 minutes per side. Mound warm potatoes atop watercress; sprinkle with green onions. Spoon vinaigrette over potatoes.