In a medium skillet, cook the bacon over medium-high heat until just crisp. Drain on paper towels and crumble when cool enough to handle. Set aside.
In a small bowl, whisk together the oil, vinegar, salt, pepper and garlic.
Wash the spinach leaves very well and spin dry. Tear the leaves into small pieces and place in a medium salad bowl. Add the mushrooms and bacon, and toss to combine. Pour 2/3 cup of the dressing over the salad (reserve the rest for another use) and toss again. Serve immediately.
Adapted from "Georgia Cooking in an Oklahoma Kitchen" by Trisha Yearwood (c) Clarkson Potter 2008