Wash and remove any sprouts or bad spots from the potatoes. If using thin-skinned potatoes(e.g. red, new, or Yukon gold potatoes), leave the skin on. If using thicker-skinned potatoes (e.g. Russet), peel them.
Cut the potatoes into 3/4 inch dice. Place in a microwave-safe dish, add water to about 1-inch deep. Cover and microwave until the potatoes are almost done. Time varies depending on amount of potatoes and strength of microwave oven but takes about 10 minutes for 2 large servings.
While the potatoes are cooking, melt the butter in a large skillet over medium heat. Chop the onions and saute in the melted butter, stirring occasionally to promote even cooking.
When the potatoes are soft, much of the water will have been absorbed. Use a slotted spoon to add the potatoes to skillet with the onions, leaving as much water behind as possible. Let the potatoes saute for about 2 minutes to brown slightly on one side.
Finely chop the parsley (and, optionally, zest the lemon). Add these to the skillet and toss to mix thoroughly. Allow this to heat through for about 2 minutes. Taste, and season with salt and pepper.
While the potatoes are heating through, you have just enough time to heat some pre-cooked smoked sausage in the microwave. Offer a dollop of mustard with the sausage and dinner is served.