Heat the olive oil in a large skillet over medium heat. Add the sausage and cook 2-3 minutes per side until beginning to brown. Remove the sausage from the pan, set aside and cover to keep warm.
In the same pan, stir in the onions, coat them with the olive oil and sprinkle them with the sugar. Let the onions stand, without stirring for 2-3 minutes until beginning to brown. Gently turn the onions and again, allowing them to stand and brown. Continue cooking until they are evenly golden brown. Stir in the Balsamic vinegar and cook another 1-2 minutes. Season to taste with salt and pepper. Return the sausage to the pan to warm if needed and serve.