Combine baby spinach leaves with stems cut off and onions to bowl and mix together.
Add the 2 TBS of oil to pan. Once heated, add the mushrooms and salt to taste and cook until they start to release their moisture and begin to shrink and they darken in color. This will take about 7-10 minutes.
With a slotted spoon, remove the mushrooms from the pan and toss with the spinach and onions.
In a separate skillet cook the chopped bacon over medium-low heat until crispy, about 5-8 minutes. Remove from the pan with a slotted spoon and set aside on paper towel to remove any extra bacon grease.
Reserve about 1/4 cup of the bacon fat and remove the rest from the pan. Add the vinegar, honey and mustard. Bring to a simmer and whisk to combine. Add salt and pepper to taste and then pour the warm dressing over the spinach, mushrooms and onions.
Top with bacon and toss to combine dressing and all ingredients.