Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain bacon slices on paper towels, reserving about 1 tablespoon bacon drippings in the skillet. Crumble bacon.
Heat reserved bacon drippings over medium heat; cook and stir onion in hot bacon drippings until slightly tender, 5 to 10 minutes.
Combine vegetable oil, sugar, vinegar, ketchup, Worcestershire sauce, and salt together in a saucepan; bring to a boil. Reduce heat and simmer until sugar and salt are dissolved, about 5 minutes. Add bacon and onion and simmer over low heat until dressing is blended, 3 minutes.
Mix spinach, mushrooms, and eggs together in a large bowl. Pour dressing over spinach mixture; toss to coat.
