Remove the sausage from its casing.
Pre-heat the olive oil in a large pan over medium heat.
Cook the sausage, until no longer pink, in batches.
When finished, use a slotted spoon or a spatula to strain the cooked sausage and place in a slow-cooker.
In a clean pan, preheat the olive oil and butter over medium heat.
Add the onions, and cook, stirring often, until golden brown around the edges and transclucent in the middle, 10-15 minutes.
Add the garlic, stir, and cook for a few minutes, until aromatic.
Add the bell peppers, stir, and sprinkle with salt. Cook, stirring often, for a few minutes.
Add the tomatoes and stir well. Cook for a few minutes.
Add the tomato sauce, stir, and let simmer, uncovered, for a few minutes. Stir often.
Pour the vegetable and tomato sauce mixture over the sausage in the slow cooker, and stir well. It will look somewhat dry.
Cook on low for 1 hour or on high for 30 minutes. Refer to Technicalities for more information on times, as they may vary.
When finished, stir well and serve with toppings of your choosing. Enjoy!