Place spinach in a big bowl and add red onion. Lightly salt the spinach leaves and red onion and toss to coat. Set in refrigerator to chill.
Meanwhile, cook the bacon pieces in a skillet of medium high heat. When cooked, drain on paper towels and set aside.
In a small skillet, toast nuts over medium heat until fragrant, about 3 minutes, stirring occasionally and set aside.
Cut pear into thin slices and peel tangerine and separate into segments, removing any pithy strings.
In a small bowl, add the Dijon mustard, balsamic and a pinch of salt, stirring to combine well. Whisk in the olive oil and taste for proper balance of acid and oil, adding more balsamic vinegar or oil to your taste.
Dress the spinach and red onions with 2/3 of the dressing and toss well to coat.
To plate, put dressed spinach and red onions on the plate, top with sliced pear and tangerine segments, sprinkle with toasted nuts, crispy bacon and top with cheese crumbles or a chunk of cheese. Drizzle with remaining dressing and freshly ground black pepper.