Sear the sausages (6 to 8 minutes). Heat the oil in a large skillet over medium-high heat. Add the links in a single layer and turn them every minute or two. They should sizzle steadily and turn a deep golden brown on most sides. They don’t need to be cooked through.
Layer the slow cooker. Spread the onion slices on the bottom of the slow cooker, pour in the stock, and lay the seared sausages on top.
Mix the glaze. In a bowl, whisk the honey, sriracha, soy sauce, vinegar, and garlic. Pour about two thirds of it over the sausages and save the rest in the refrigerator for the finish.
Cook on low (about 3 hours). Cover and cook on low. Start checking at 3 hours with an instant-read thermometer in the thickest part of one link. Raw pork sausage is safe at 71°C (160°F). Thick links may need up to 4 hours. I haven’t tested every slow cooker, which run hotter or cooler, so trust the thermometer over the clock.
Reduce the glaze (about 8 minutes). Move the sausages to a plate. Strain the juices into a saucepan, skim off the fat, and add the reserved glaze. Boil gently over medium heat until the sauce looks glossy and coats the back of a spoon, and a line drawn through it holds for a second or two.
Coat the sausages (2 to 3 minutes). Return the sausages to the skillet or saucepan and turn them in the glaze over medium heat until they look shiny and sticky. For a lightly charred finish, put them under a hot broiler for 3 minutes, watching closely, since honey burns fast.
Serve. Spoon any remaining glaze over the top, then sprinkle with green onions and sesame seeds.