Preheat grill.
Add vinegar, water, sugar, and salt for the pickles to a medium saucepan. Bring mixture to a boil, stirring occasionally, over high heat.
Remove pan from heat and add cucumber slices. Cover with a double layer of paper towels, pressing down until the towel is completely wet with the liquid.
Let rest for 10 minutes.
Add cucumber slices to a mason jar, then fill with liquid discarding any remaining liquid.
Seal and place in the refrigerator until ready to serve.
In a large bowl, combine beef, Sriracha, salt, pepper, and Chinese five spice in a medium bowl. I like to use my hands to combine the meat mixture.
Divide meat mixture in half and form into patties.
In a small bowl, whisk together all ingredients for the glaze. Set aside.
Place burgers on the grill, cover, and cook until browned on the bottom, about 3 to 4 minutes. Flip patties and brush with miso glaze. Cook for 3 minutes, turn again, and brush the top and sides with the miso glaze.
Grill until patties are cooked through to desired temperature (it took me an additional 4 to 5 minutes). Continue to flip every minute or so, glazing between each flip, for the remaining cooking time.
Spread a thick layer of mashed avocado on the bottom of each toasted half of the bun.
Place the burgers on top, following by grilled yellow onion slices, Asian cucumber pickle slices, and a pinch of red pepper flakes.
Top with the other halves of the buns and serve immediately.