In a small pot, combine rice, 1 ¼ cups water and a large pinch of salt. Bring to a boil, then cover and reduce heat to low. Cook until rice is tender, 15-20 minutes. Keep covered off heat until ready to serve.
Meanwhile, clean and dry all produce. Peel and mince garlic. Zest and quarter lime. Cut ends off and halve the cucumber lengthwise; slice crosswise into thin half-moons. Halve, peel, and medium dice onion. Trim, peel, and grate carrot.
In a small bowl, combine mayonnaise, as much sriracha as you’d like (add more for extra spicy!), a pinch of the minced garlic, and a squeeze of lime juice. Season with salt and pepper.
In a medium bowl, combine cucumber, ¼ tsp sugar a pinch of salt, and juice from 2 lime wedges.
Heat a drizzle of oil in a large pan over medium-high heat. Add onion and cook, stirring, until softened, 4-5 minutes. Add beef, remaining garlic, and 2 tsp sugar. Cook, breaking up meat into pieces, until browned and cooked through, about 5 minutes. Stir in soy sauce. Turn off heat; taste and season with salt and pepper.
Fluff rice with a fork and stir in lime zest and 1 TBSP butter; divide between bowls. Arrange beef, grated carrot, and pickled cucumber on top. Top with a squeeze of lime juice. Drizzle with sriracha mayo.
**TIP: To get a more authentic Bahn Mi, allow the carrots and cucumber to marinate in 3 tbsp rice wine vinegar and 1 tsp salt and 1/2 tsp sugar