Mix vinegar, sugar, water, butter, and salt together in a saucepan; bring to a boil. Cook and stir until sugar dissolves and mixture is bubbling, about 1 minute. Remove from heat and cool to room temperature, about 1 hour.
Whisk egg into dressing; return saucepan to low heat and simmer until dressing thickens, 5 to 10 minutes.
Combine spinach, mushrooms, mandarin oranges, almonds, bacon, red onion rings, and croutons in a large bowl; pour dressing over salad. Toss to coat.
All done! Now take a photo, rate it, and share your accomplishments!