Step 1Place the spinach in a large bowl; cover with a damp paper towel. (The spinach can be refrigerated several hours or overnight.) Combine the vinegar, water, oil, salt and pepper to taste in a small bowl. Set aside.
Step 2Cook the bacon in a medium nonstick skillet over medium-high heat until crisp, about 4 minutes. Use a slotted spoon to transfer the bacon to a paper towel. Pour off the fat but do not clean or wipe the pan. Add the vinaigrette to pan. (This can be done several hours ahead and kept at room temperature.)
Step 3To serve, bring the vinaigrette to a boil; add the onion. Pour the hot vinaigrette over the spinach. Toss several times to coat the greens. Add freshly ground pepper and the bacon. Toss. Taste; adjust the seasoning. Serve immediately.