Cut bacon slices into eighths and fry until crisp. Drain on paper towels. Reserve 2 Tbsp. of the drippings.
Meanwhile, wash lettuce and tear into bite-sized pieces. Remove stems from spinach and combine with lettuce in salad bowl. Add the onions. Cover and refrigerate until just before serving.
Add sugar, salt, pepper, vinegar and water to the reserved bacon drippings in the pan. Set aside until ready to serve.
At serving time, bring dressing mixture to boiling. Stir and pour over the lettuce-spinach mixture. Toss. Top with bacon bits.
Sprinkle with croutons and/or other desired toppings. Serve immediately.