Whisk together garlic, lemongrass, Hoisin sauce, soy sauce, fish sauce, brown sugar, and Sriracha. Set aside.
Heat a large skillet or wok over medium-high heat. Add cooking oil. When oil begins to shimmer, add pork and saute, breaking it apart until pork is just starting to turn opaque, 4 to 5 minutes.
Pour sauce over pork and continue cooking until sauce cooks down and starts to caramelize, 6 to 8 minutes more. If using chilis, stir them in the last minute of cooking.
Assemble bowls with a bed of lettuce topped with rice. Add cucumbers on one side and top with pork.
Garnish bowls with fresh cilantro and / or fresh mint and lime wedges (be sure to squeeze a few lime wedges over top of the pork before serving).