Generously salt and pepper pork shoulder. Place in slow cooker.
Mix marinade ingredients in small bowl. Rub marinade all over pork shoulder with hands.
Scatter onion and jalapeno over the top.
Cover and cook until tender and falling apart, about 4 hours on high setting or 6-7 hours on low setting. This will depend on the thickness of the meat, so feel free to check for tenderness after the minimal time allowance.
Open lid and shred pork with two forks, directly into the slow cooker. This will ensure the pork soaks up all the sauce. Let stand 10 minutes.
Arrange top rack in oven so that it's 3-4 inches from the heat source. Set oven to broil and pre-heat for 10 minutes (while the shredded pork stands and absorbs the sauce).
Using a slotted spoon, remove pork onto foil-lined baking sheet. Drizzle leftover sauce from the slow cooker over the pork. Broil at high heat until the pork is browned with crispy edges and charred bits, about 4-6 minutes. Watch carefully so that it doesn't burn too much.
Transfer to serving plate and garnish with cilantro or green onion. Serve immediately or at room temperature.