Wash the spinach leaves thoroughly. If the leaves are large, give the spinach a rough chop. Pat the spinach dry with paper towels or use a salad spinner. Refrigerate the cleaned spinach until you are ready to toss the salad.
In a heavy skillet over medium-low heat, fry the diced bacon until it’s browned and crisp, turning frequently. With a slotted spoon, remove the bacon to paper towels to drain.
To the skillet, add the sugar, vinegar, and dry mustard; bring to a boil. Remove from the heat and add salt and pepper, to taste. Serve right away or refrigerate until it’s time to serve.
Before serving, reheat the sauce mixture; drizzle over the spinach leaves and toss lightly. Serve garnished with the bacon, sliced red onion, and sliced hard-boiled eggs.