Heat a large sauté pan over medium heat, add the oil and heat through. Add 2 tablespoons of the the butter (28g) and allow it to melt, then add the onions and peppers. Cook, stirring occasionally, until the vegetables are soft, about 6 minutes.
Add the tomatoes and continue to cook, stirring occasionally until they break down and burst, becoming jammy, about an additional 6 minutes. Season with salt then set aside and cover.
Spread the sausage in a thin layer over one side of each slice of bread, pressing to adhere.
Return the pan to medium heat. Working in batches, add the bread to the pan, sausage-side down, pressing down with a metal spatula to sear the meat until completely cooked, about 2 to 3 minutes.
Flip the toast over bread side down and add a cube of the remaining butter to the pan so the bread cooks in the butter and becomes crispy, about an additional minute.
Remove the sausage toast to a plate. Repeat with remaining bread. Spoon the cooked onion and pepper mixture over the sausage. Cut each toast into thirds and garnish with Parmesan, chives and parsley. Serve immediately.
Video
Notes
If you prefer less spice, I recommend swapping the hot Italian sausage for mild Italian sausage.