Place the spinach in a large bowl and leave at room temperature. Bring a medium pot of water to a boil over medium-high heat. Use a slotted spoon to gently lower the eggs into the water. Boil until both the whites and yolks are set (hard-boiled), about 10 minutes. Transfer to a small bowl filled with ice water and cool completely. Once cool, peel the eggs and cut each into quarters.
Meanwhile, put the bacon in a large skillet and set over medium heat. Cook, stirring occasionally, until golden brown and crisp, 6 to 8 minutes. Transfer the bacon with a slotted spoon to a paper towel-lined plate to drain. Remove half of the bacon fat to a small bowl and reserve.
Return the skillet and remaining bacon fat to medium heat. Add the mushrooms, garlic, 1/4 teaspoon of salt and a few grinds of black pepper to the same skillet. Cook, stirring occasionally, until tender, 3 to 4 minutes. Pour in the reserved bacon fat, vinegar, mustard and honey and whisk until smooth and emulsified. Taste and season with more salt and pepper.
Pour the hot bacon dressing over the spinach and toss until evenly coated and slightly wilted. Transfer to a platter and top with the eggs, red onion and reserved bacon.
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