In a large frying pan or sauté pan heat the olive oil over medium heat. Brown the sausages on all sides, about 8 minutes. Sausages will not be cooked through at this point. Remove sausages from pan and set aside.
In the same pan add the sliced onion and peppers and a pinch of salt. Sauté over medium heat, partially covered, stirring often until softened, about 6-8 minutes. Add the garlic and red pepper flakes if using. Saute for about a minute until garlic is fragrant. Add the red wine and deglaze bottom of the pan. Cook for about 2 minutes to burn off the alcohol, stirring occasionally. Add the tomatoes and stir to combine. Simmer on low heat about 5 minutes.
Option 1 : Place the sausages in a slow cooker and top with the tomato sauce mixture. Cook on LOW for about 2 hours to finish cooking the sausages.
Option 2 : Add the sausages back into the sauté pan with the tomato mixture and simmer on low for about 30 minutes, partially covered, to finish cooking the sausages through.
Serve sausages and peppers alone as an appetizer or with a side dish of pasta. Top with fresh chopped basil before serving.
Enjoy!