Place onions and mushrooms into a large baking pan.
Add garlic, olive oil, salt, lemon juice, mustard into a blender and blend.
Taste Mustard sauce to see if more lemon juice is needed.
Drizzle 1/2 the sauce over mushrooms and onions. Top with Rosemary and bake for 35 minutes.
Wrap potatoes in foil and bake until soft. Peel potatoes once they have baked.
Cut potatoes into smaller size and place into a bowl. Add 2 tablespoons of Lemon juice and mix.
Add chopped parsley and onion, drizzle with the rest of the mustard sauce and mix.