Cook bacon until crispy. Drain on paper towel, but do not remove grease from skillet.
Place whole slices of red onion into hot skillet and sear in the bacon grease until just starting to turn transparent, but still firm. Stir in the vanilla paste. Place on paper towel to remove residual grease. Slices can be sectioned at this time.
We kept our mushrooms raw, but if you prefer yours cooked, sauté in the same skillet for a minute or so.
Toss the spinach, onion, and mushrooms together in a bowl.
Combine all the dressing ingredients together in a container with a lid and shake until well mixed.
When ready to serve, pour desired amount of dressing over the spinach and toss.
