Mix onion with vinegar and salt, stir to thoroughly combine, set aside.

Heat 1 Tbsp olive oil in a large skillet over medium–high heat. Add the sausage and cook, breaking it up into smaller pieces as you go. Stir occasionally, until the sausage is browned and crispy (about 8-10 minutes).

Remove sausage from skillet to a paper towel lined plate, make sure to leave any fat in the skillet.

Add 2 Tbsp olive oil to the skillet. Once heated (1 minute), add the brussels sprouts cut-side down. Season with salt and pepper. Let the brussels cook without stirring for about 5 minutes until the brussels sprouts are well browned. Stir to brown the brussels sprouts all over (another 5-8 minutes).

Add the Italian sausage to the brussels sprouts and stir to combine.

Serve topped with pickled onions.