In a small saucepan, add the eggs and a few pinches of salt. Cover the eggs with cold water and bring to a boil over medium heat. Remove the pan from the heat and cover with a lid for 10 minutes. Remove from water and cool in a bowl of iced water. Peel and set aside.
In a large saute pan over medium heat, add the oil. Add the onions and bacon and saute until the bacon is crisp. Stir in the vinegar, mustard, sugar, black pepper and salt and cook for 1 minute. Set aside and keep warm.
In a large serving bowl add the spinach, avocado and the lemon juice to keep avocado from turning brown. Season with salt and pepper, to taste. Cut the eggs into quarters and add to the bowl. Ladle desired amount of dressing on the salad and serve.