Heat a dry large nonstick skillet or wok over high. Add 1 Tbsp. vegetable oil, then 1 lb. hot Italian sausage, casings removed. Using a wooden spoon, break up sausage into bite-size pieces and cook, stirring every couple of minutes, until sausage is mostly cooked through with some golden brown edges, 6–8 minutes. Using a slotted spoon, transfer sausage to a plate. Pour fat in skillet into a small heatproof bowl or measuring glass.
Add 1 Tbsp. fat back to pan and cook 1 medium onion, thinly sliced, tossing every 30 seconds or so, until softened but not floppy, about 3 minutes. Stir in 4 garlic cloves, thinly sliced, and 1 small jalapeño, thinly sliced, and cook, stirring often, until softened, about 2 minute. Transfer to plate with sausage.
Heat another 3 Tbsp. fat (or vegetable oil if you’ve run out of fat) in same pan. Add 1 lb. brussels sprouts, halved, season with kosher salt, and toss to combine. Arrange brussels sprouts so each piece has as much contact with pan as possible. Let cook, undisturbed, until golden brown around edges underneath, about 3 minutes. Toss and pour in 2 Tbsp. soy sauce and ½ cup water. Reduce heat to medium, cover pan, and cook until brussels sprouts are just tender, about 5 minutes. Remove pan from heat and return sausage and onion mixture back to pan if you have room. Add 1 Tbsp. sherry vinegar or red wine vinegar and 2½ tsp. five-spice powder and toss to combine. (If you don’t have enough room in the pan, toss everything together in a large bowl.)