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In a jar add the whole grain mustard, dijon mustard, 2 tablespoons of the red wine vinegar, salt, pepper and pepper and olive oil. Shake vigorously. Set aside.
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Put the potatoes in a large pot of cold, well-salted water. Bring to a boil, reduce to a simmer and cook until tender when pierced, about 15 minutes. Drain well and transfer to a large bowl to cool slightly. Sprinkle with 1 tablespoon red wine vinegar.
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In a skillet, warm 1 table spoon olive oil. Add onion and cook for about 2 minutes then add garlic- cook for additional minute. Add diced tomatoes and cook for about 1-2 minutes. Remove from heat.
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Add some of the vinaigrette to the potatoes and toss. Add the tomato and toss again. Taste and adjust the salt and black pepper.
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In a salad bowl, toss together the arugula and watercress. Then top with potato and tomato mixtures. toss to serve.