Combine onion, mayonnaise, Sriracha, and lemon juice in a small bowl; stir with a whisk. Combine tomato, avocado, and bacon in a medium bowl; toss to combine. Divide spinach among 4 plates; top evenly with avocado mixture. Drizzle with Sriracha mayo; sprinkle with pepper.
If you like sriracha or just spicy in general, this is an amazing riff on a BLT salad!
Glad I tasted the Sriracha mayo before serving -- too hot for some of us -- and we like hot.
