1Stem the spinach and place in a large bowl. Cover with a paper towel and refrigerate while preparing the rest of the salad.
2In a 9-inch skillet, cook the bacon until crisp. Transfer to a paper towel-lined plate. Pour off the fat.
3Put the skillet back over medium-high heat and add a tablespoon olive oil. Put the mushrooms into the skillet and saute until they are soft and have given up their liquid. Remove with a slotted spoon to a plate.
4Put the remaining 2 tablespoons olive oil, lemon juice, mustard and salt into the pan and mix with a spatula to dissolve the mustard. Put the bacon and mushrooms into the pan and cook 30 seconds to thicken the juices. Pour over the spinach, season with pepper and add the Parmesan.