Thick-sliced bacon, along with its melted fat, provide not only flavor but also the hot dressing to wilt the greens in this spinach salad.
Toss spinach and onion in a large bowl. Heat a large skillet over medium heat. Add olive oil and bacon, and cook, stirring occasionally, until well browned.
Transfer bacon to a paper-towel-lined plate to drain. Remove and reserve 4 tablespoons of the bacon fat from the skillet, and place the remainder back over medium heat. Add the shallot, and cook until softened, 1 to 2 minutes. Stir in vinegar, sugar, honey mustard, salt, and pepper. Cook, stirring occasionally, until sugar is dissolved and dressing is hot, about 1 minute.
Pour dressing over spinach, add reserved bacon, and toss well. Heat 2 tablespoons of the reserved bacon fat in the same skillet. Add half the bread cubes, and toss to coat them evenly. Cook until bread cubes are golden, about 3 minutes.
Transfer to a paper towel, and repeat with remaining bread cubes, adding remaining 2 tablespoons bacon fat to pan. Add to salad, and toss well. Serve immediately.
Three bunches may seem like a lot of spinach, but the hot-bacon dressing wilts it substantially.