Place spinach into a large bowl. Cook bacon in a large frying-pan over medium heat for about four minutes, or until it is just crispy. Add Canadian bacon to the frying-pan and cook for two more minutes, stirring frequently. Remove meat from pan and place on a plate lined with paper towels. Drain any remaining fat from the frying-pan. Add olive oil and onions to the skillet and cook for about two minutes, or until onions soften slightly. Add mushrooms to the pan and cook, stirring frequently, for two more minutes. Put onions and mushrooms on top of the spinach. Add apple cider and vinegar to the frying-pan 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 120ml. 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. Sprinkle the bacon on top and serve.