In a medium saucepan over high heat, combine rice and 2 cups water; season with salt. Bring to a boil, then cover and reduce heat to medium-low. Simmer until rice is tender and water is absorbed, about 17 minutes.

Meanwhile, in a medium bowl, whisk sour cream and sriracha until smooth. If needed, add water, a few drops at a time, until sauce is a good consistency for drizzling. Refrigerate until ready to use.

In another medium bowl, whisk brown sugar, soy sauce, cornstarch, vinegar, sesame oil, and ½ cup water until smooth. Let sit at room temperature until ready to use.

Remove dark green parts from scallions and thinly slice on a diagonal; set aside for serving. Thinly slice white and light green parts.

In a large nonstick skillet or wok over high heat, heat vegetable oil. Cook garlic, ginger, and white and light green scallion parts, stirring frequently, until light golden, about 1 minute.

Add beef and cook, breaking up into smaller pieces, until just cooked through and starting to brown, about 5 minutes.

Add broccoli and cook, stirring frequently, until bright-green, about 2 minutes.

Add carrots and reserved soy sauce mixture. Cook, stirring frequently, until sauce thickens and vegetables are crisp-tender, 3 to 4 minutes more.

Divide rice among bowls. Spoon beef mixture over top. Drizzle with sriracha-sour cream. Sprinkle with reserved dark green scallion parts and sesame seeds.