Scrub potatoes under cold running water, steam potatoes until tender. Cool and slice.
In a large non-reactive bowl, combine and whisk the marinade ingredients. Add the mushrooms and marinate 30 minutes.
Remove the mushrooms from the marinade and grill or bake until cooked through. The mushrooms should be dark in color and have a deep rich flavor. Slice mushrooms.
In skillet over medium heat, heat the 1 tablespoon olive oil. Add sweet onion strips and cook and stir until deep golden in color.
In a small bowl, whisk salad dressing ingredients together.
In a large non-reactive bowl, combine potatoes, mushrooms and onions. Cover with the dressing. Adjust seasoning with salt and pepper. The mixture should be well covered with the dressing, but should not appear oily.
Nutritional Analysis per serving: 398 calories, 5 g protein, 29 g fat (65% calories from fat)