Place spinach into a large bowl. Cook bacon in a large skillet over medium heat for about 4 minutes, or until it is just crispy.
Add Canadian bacon to the skillet and cook for 2 more minutes, stirring frequently.
Remove meat from pan and place on a plate lined with paper towels.
Drain any remaining fat from the skillet.
Add olive oil and onions to the skillet and cook for about 2 minutes, or until onions soften slightly.
Add mushrooms to the pan and cook, stirring frequently, for 2 more minutes. Put onions and mushrooms on top of the spinach.
Add apple cider and vinegar to the skillet and turn the heat up to medium-high. Stir to scrape up any bits that are stuck to the bottom of the pan and cook for 8 to 10 minutes or until cider is reduced to about 1/2 cup.
Whisk in mustard, salt and pepper, to taste.
Pour warm cider dressing over the mushrooms and spinach and toss until the vegetables are well coated.