1Grate the onion into a mixing bowl and mix the salt, pepper and mustard into it. Moisten this paste with the vinegar. With a wire whisk, gradually beat in the oil. Continue beating until the dressing reaches the consistency of a light mayonnaise. Stir in lemon juice.
2Slice mushrooms into a salad bowl and toss with 2 tablespoons of dressing. Add spinach and bacon to the mushrooms. Add enough dressing to coat thoroughly.