Place a large skillet over medium heat and add the chopped bacon.
Cook, stirring occasionally, until the bacon is brown and crispy. Use a slotted spoon to transfer the bacon to a paper towel-lined plate, leaving the rendered fat in the skillet.
While the bacon is cooking, place the rinsed spinach and thinly sliced red onion in a large salad bowl.
Remove the skillet from the heat.
Whisk the mayonnaise directly into the warm bacon fat until a smooth, creamy dressing forms.
Season with a pinch of salt and pepper.
Pour the warm dressing over the spinach and onions in the bowl.
Add the sliced eggs and half of the crispy bacon, then toss gently to combine everything.
Garnish with the remaining bacon bits and serve immediately.
Add a teaspoon of Dijon mustard or apple cider vinegar to the dressing to add a tangy counterpoint to the rich bacon fat and mayonnaise.
