Place spinach in a large, wide salad serving bowl. Scatter with mushrooms, red onion (see above for a different, mellower way to add the onions) and slices of hard-boiled egg.
In a large skillet, fry bacon bits over medium-high heat until they’re brown and crisp and have rendered their fat. Use a slotted spoon to scoop them out of the skillet and spread them on a piece of paper towel briefly before sprinkling them over the salad. Pour out all but two tablespoons of hot bacon fat from the skillet.
Reheat over medium and quickly whisk in the red wine vinegar, honey and Dijon. Pour over entire salad and season salt and pepper. Toss gently and serve hot.