Heat 2 tablespoons olive oil over medium-high heat in a wide, heavy pan. Add sausage rounds in a single layer. Brown 3–4 minutes per side until deeply golden. Remove and set aside.
Reduce heat to medium. Add remaining 2 tablespoons olive oil. Cook sliced onion 8–10 minutes until soft, translucent, and just beginning to turn golden.
Add the smashed garlic cloves and cook, stirring, for 1 minute until fragrant. Do not let it color.
Add all peppers — sweet red, green, and hot. Cook over medium heat for 10 minutes, stirring occasionally, until softened and beginning to blister.
Push the peppers to the edges. Add tomato paste to the center of the pan. Cook 2 minutes undisturbed until it darkens to brick color. Stir through the peppers.
Pour in the red wine and scrape up all the browned bits from the bottom. Let reduce by half, about 3 minutes.
Add crushed tomatoes, oregano, bay leaves, and a generous amount of freshly ground black pepper. Stir to combine.
Return the sausage rounds to the pan, pressing them into the sauce so they are mostly submerged. Bring to a gentle simmer.
Reduce heat to low. Set the lid slightly ajar. Cook 30–35 minutes, stirring every 10 minutes, until the sauce has thickened, the fat has rendered and integrated, and the peppers are completely tender.
Discard bay leaves. Taste and adjust salt. Scatter parsley over the top if using. Serve directly from the pan with plenty of crusty bread.