Place the bacon pieces in a medium nonstick skillet.
Cook the bacon over medium heat for 8 to 10 minutes, stirring occasionally, until crisp.
While the bacon is cooking, prepare the dressing base.
In a large mixing bowl, whisk together the cider vinegar, honey, Dijon mustard, salt, and freshly ground black pepper.
Set the dressing base aside.
Using a slotted spoon, transfer the cooked bacon to a paper towel-lined plate.
Carefully pour the rendered bacon fat into a heatproof bowl.
Return 4 tbsp of the bacon fat to the skillet.
Discard the remaining bacon fat or reserve it for another cooking use.
Add the finely chopped shallots to the skillet with the reserved bacon fat.
Cook the shallots over low heat for 1 to 2 minutes, stirring frequently, until softened.
Do not allow the shallots to brown.
Add the warm bacon fat and softened shallots from the skillet to the vinegar mixture.
Whisk until the dressing is fully combined.
Add the baby spinach and sliced mushrooms to the bowl with the warm bacon dressing.
Toss gently until the spinach and mushrooms are evenly coated.
Taste and adjust the seasoning with additional salt and black pepper, if needed.
Divide the dressed spinach and mushrooms evenly among 4 plates or bowls.
Top each serving with the sliced or wedged boiled eggs.
Divide the cooked bacon evenly among the salads.
Serve immediately while the dressing is warm and the bacon is crisp.