Cook Rice and Prep:
Wash and dry all produce. In a small pot, combine rice with 3/4 cup water. Cover, bring to a boil, then reduce to a simmer. Cook until water is absorbed and rice is tender, about 15 minutes. Keep covered off heat.
Pickle Cucumber:
In a medium bowl, combine 2 1/2 tsp vinegar, 1/2 tsp sugar, and a few pinches of salt. Trim ends from cucumber. Using a peeler, shave cucumber into ribbons, rotating until the seedy core is reached; discard core. Toss ribbons in the vinegar mixture and set aside.
Cook Carrots:
Heat a drizzle of oil in a large pan over medium-high heat. Add carrots and sauté for about 1 minute until tender but slightly crisp. Season with salt and pepper. Transfer to a plate.
Cook Beef:
In the same pan, heat another drizzle of oil over medium-high heat. Add scallion whites and cook for 30 seconds. Add ground beef, season with salt and pepper, and break up into pieces. Cook until browned, about 5-7 minutes. Stir in half the sesame seeds and the remaining vinegar. Cook for 30 seconds, then add bulgogi sauce and bring to a simmer. Remove from heat and season to taste.
Make Sriracha Crema:
In a small bowl, mix sour cream and sriracha to taste. Stir in water 1 tsp at a time until drizzling consistency is reached. Season with salt.
Finish and Serve:
Fluff rice with a fork and stir in butter. Season with salt and pepper. Divide rice between bowls. Arrange beef, carrots, and cucumber in sections. Drain cucumber if needed. Drizzle with sriracha crema. Garnish with scallion greens and remaining sesame seeds.