This hearty one-pan dish is perfect for chilly nights.
In a cast-iron skillet over medium heat, brown sausage in oil on all sides, about 20 minutes. Remove from pan, and keep warm. Do not wash pan.
Cook onions in same pan over medium heat, stirring often, until tender and translucent, about 15 minutes.
Add vinegar and cook 1 minute. Add 1 cup water, salt, pepper, and whole rosemary leaves, and cook over medium-high heat, scraping bottom of pan, until reduced by half.
Lower heat to a simmer, and return sausage to pan. Simmer together with onions for 5 to 10 minutes. Serve immediately.