Slice the onions and bell peppers, thinly slice the garlic, chop the parsley, and lightly crush the fennel seeds.
Heat the olive oil in a large wide skillet or Dutch oven over medium heat. Add the sausage links and brown on all sides until well colored, 8–10 minutes. Transfer sausages to a plate.
Spoon off excess fat so about 2 tbsp remain in the pan. Add the onions and bell peppers, season with 1 tsp kosher salt and 0.25 tsp black pepper, and cook, stirring occasionally, until softened and lightly browned at the edges, 8–10 minutes.
Stir in the garlic, red pepper flakes, fennel seeds, and dried oregano. Cook until fragrant, 30–60 seconds.
Pour in the white wine and scrape up any browned bits. Simmer until reduced by about half, 2–3 minutes.
Add the crushed tomatoes and stir to combine. Return the sausages and any accumulated juices to the pan, nestling them into the peppers and onions. Sprinkle in 0.25 tsp kosher salt.
Bring to a gentle simmer, cover partially, and cook until the sausages are cooked through (160°F/71°C) and the peppers are tender, 15–20 minutes.
Uncover and simmer to thicken the sauce to a glossy, spoon-coating consistency, 3–5 minutes.
Season with the remaining 0.25 tsp black pepper and adjust salt within the measured amount as needed.
Stir in the chopped parsley and let rest off heat for 5 minutes to settle the flavors.
Serve hot, spooning sausages, peppers, and sauce onto plates.