For the dressing: Combine the oil, sugar, ketchup, vinegar, Worcestershire sauce and grated onion in a bowl, and whisk together. You can add a little water if you prefer a looser consistency.
For the salad: Place the eggs in a saucepan and cover by 1-inch with cold water. Add a pinch of salt and bring the water to a boil. Then remove the pan from the heat, cover with a lid and leave the eggs for 10 to 12 minutes. Rinse the eggs in cold water and chill them completely in the fridge. Once cool peel and halve the eggs.
Heat a skillet over medium-high heat. Cook the bacon, turning over once or twice, until cooked through, but not crisp. Drain on a paper towel and chop into lardons.
Place the spinach in a bowl with the bean sprouts, mushrooms, eggs and bacon, arranged however you would like. Drizzle with the dressing and serve with a baguette and salted butter on the side.