In a large skillet over medium heat, fry bacon until crisp, about 5 minutes. Transfer to a plate lined with paper towels.
Pour rendered bacon fat into a measuring cup. Pour off all but 1/4 cup, or add olive oil to reach 1/4 cup. Return fat to skillet over medium-high heat until shimmering.
Add onion, sugar, vinegar, and salt and pepper to taste (I like 1/2 teaspoon salt and 1/4 teaspoon pepper). Cook until onion is softened, about 3 minutes. Remove from heat.
To make the salad:
In a very large bowl, add spinach. Immediately pour over warm bacon dressing and toss to wilt. Portion the salad onto individual plates if desired and garnish with bacon and eggs.
Olive oil: If you don't render enough fat from your bacon, just add olive oil until you reach 1/4 cup.
Red onion: Minced shallot works too.
Sugar: This dressing is sweet. If you don't like overly sweet dressings, feel free to start with 1 or 2 tablespoons of sugar and increase as you see fit.
Vinegar: I always use white vinegar, but cider vinegar, red wine vinegar, or white wine vinegar are also good.
Spinach: Flat-leaf spinach is the best for salads and is usually sold in bunches or pre-washed in bags (often labeled "baby spinach"). Curly-leaf spinach, sold in crinkly bags, is tough, fibrous, and better for cooking than eating raw.
Cleaning your spinach: Wash small amounts of greens in a bowl, swishing around to loosen any dirt. Spin dry in a salad spinner or lay on a clean kitchen towel. Roll up the kitchen towel and store in the refrigerator to keep your greens fresh.