1. Place bacon in a large, deep skillet. Cook over medium high heat until evenly brown. Drain, crumble and set aside.
2. Place vinegar, red onion, mustard, and salt into the bowl of an upright blender. Cover, and puree on high until smooth. Reduce blender speed to medium-low; slowly pour in the oil and blend until incorporated and the dressing is creamy. Stir in the poppy seeds and set aside.
3. Toss the spinach in a large bowl with the crumbled bacon, Mandarin oranges, and enough dressing to moisten. Place onto salad plates and sprinkle with toasted almonds.