Wash spinach in three changes of water, drain and spin dry. Tear leaves into bite sized pieces, and combine in a bowl with sliced mushroom and onion. The Dressing: Cook bacon, reserving slices. Add flour to the fat to make a roux. Add water, bring to boil and stir with a wooden spoon to thicken. Beat an egg, and beat hot dressing into egg. Season with salt and pepper. Add vinegar first, to make it sour, to taste. Add sugar to make it sweet, to taste. Add cream, and reseason if needed Add crumbled bacon, and more bacon grease for flavor, to taste Pour the finished still hot dressing over the spinach or lettuce in a bowl, cover it and allow the dressing to wilt the salad. Toss well and serve. Notes: This dressing is served hot to warm over lettuce, or in this case, spinach and mushrooms, and is covered, to allow it to wilt the greens.