Wash and dry all produce.

In a small pot, combine rice & 1 cup water. Bring to a boil, then cover and reduce to a low simmer. Cook until rice is tender, 15-18 minutes.

Meanwhile, trim and thinly slice scallions, separating whites from greens.

In a medium bowl, combine half the vinegar (you'll use the rest later), ½ tsp sugar, and a pinch of salt. Trim ends from cucumber. Using a peeler, shave cucumber lengthwise into ribbons, rotating as you go, until you get to the seedy core; discard core. Set aside, tossing in vinegar mixture occasionally, until ready to serve.

Heat a drizzle of oil in same pan over medium-high heat. Add scallion whites and cook until fragrant, 1 minute.

Add beef and a pinch of salt and pepper. Cook, breaking up meat into pieces, until browned and cooked through, 4-6 minutes.

Stir in half the sesame seeds (save the rest for serving) and remaining vinegar. Cook for 30 seconds, then immediately turn off heat. Season again with salt and pepper.

While beef cooks, in a small bowl, combine sour cream with sriracha to taste. Stir in water 1 tsp at a time until mixture reaches a drizzling consistency. Season with salt.

Fluff rice with a fork; stir in TBSP butter (2 TBSP for 4 servings) and season with salt and pepper. Divide between bowls.

Arrange beef, carrots, and pickled cucumber (draining first) on top. Drizzle crema over everything. Garnish with scallion greens and as many remaining sesame seeds as you like. Serve.