PreparationFor pickled spring onions: Combine onions, vinegar, sugar, salt, and 1 tablespoon water in a small bowl and toss to combine; let stand for 30 minutes. For maple bacon and spicy mayo: Preheat oven to 350°F. Place bacon on a foil-lined large rimmed baking sheet; brush both sides with syrup. Bake until bacon begins to crisp but is still pliable, 20-25 minutes. Mix mayonnaise and Sriracha in a small bowl to combine; set aside. For fried eggs and assembly: Spread 1 side of bread slices with plain mayonnaise. Heat a large skillet over medium-low heat. Working in batches, cook bread, mayonnaise side down, until brown and crisp, about 3 minutes. Wipe out skillet. Melt butter in same skillet over medium heat; crack eggs into skillet. Cook, occasionally basting with butter in skillet, until whites are set, about 3 minutes. Season with salt and pepper. Spread untoasted side of each bread slice with spicy mayo. Build sandwiches with bread, bacon, eggs, pickled spring onions, and arugula.
PreparationFor pickled spring onions: Combine onions, vinegar, sugar, salt, and 1 tablespoon water in a small bowl and toss to combine; let stand for 30 minutes. For maple bacon and spicy mayo: Preheat oven to 350°F. Place bacon on a foil-lined large rimmed baking sheet; brush both sides with syrup. Bake until bacon begins to crisp but is still pliable, 20-25 minutes. Mix mayonnaise and Sriracha in a small bowl to combine; set aside. For fried eggs and assembly: Spread 1 side of bread slices with plain mayonnaise. Heat a large skillet over medium-low heat. Working in batches, cook bread, mayonnaise side down, until brown and crisp, about 3 minutes. Wipe out skillet. Melt butter in same skillet over medium heat; crack eggs into skillet. Cook, occasionally basting with butter in skillet, until whites are set, about 3 minutes. Season with salt and pepper. Spread untoasted side of each bread slice with spicy mayo. Build sandwiches with bread, bacon, eggs, pickled spring onions, and arugula.