Place the bacon in a medium nonstick skillet and fry over medium heat, stirring occasionally, until crisp, 8 to 10 minutes.
Using a slotted spoon, transfer the cooked bacon to a paper towel-lined plate. And then chop to add to the salad.
Pour the bacon fat into a heatproof bowl, then return 2 tablespoons of the bacon fat to the skillet. Add the shallots to the skillet and cook over low heat, stirring frequently, until softened, 1 to 2 minutes. Do not brown.
Add the cider vinegar and scrape the bottom of the pan.
Add the mustard, salt, and pepper and bring to a boil. Reduce by half.
Meanwhile, assemble the salads. This can make 2 large salads or 4 small salads
Once the dressing has reduced by half, remove from the heat and slowly whisk in the oil.
I like to toss the salad with the hot dressings, allowing it to wilt the spinach. But this salad is also good if you let the dressing cool.
If you soak the onion in ice water for a few moments, it helps make them crisp and removes the strong, pungent taste.
All of the salad toppings are optional, and you can substitute pears for apples and add goat cheese.