To make the pickled onions: Place the onions in a clean, sealable quart-size jar. In a small pot, combine the vinegar, water, sugar, and salt and bring to a boil over high heat, stirring to dissolve the sugar. Let cool for 5 minutes.
Pour the pickling liquid over the onions and let cool for 1 hour. Cover and transfer the jar to the refrigerator until ready to use. Pickles will keep in the refrigerator for up to two weeks.
To make the spicy mayonnaise: In a small bowl, stir together mayonnaise and sriracha. Season with salt.
To make the poke bagel: In a small bowl, toss the tuna with the soy sauce, sesame oil, and vinegar. Season with salt.
Spread a heaping tablespoon of the spicy mayonnaise on each bagel half. Divide the tuna and avocado on top.
Garnish with the sesame seeds and pickled red onions, drizzle the remaining spicy mayonnaise on top, and sprinkle with microgreens. Serve immediately.