Preheat oven to 180ºC/350ºF. Arrange sausages on a tray and bake for 25 mins until golden brown. Remove and slice into ¼” pieces.
Heat 1 tsp oil in a saute pan over medium heat. Add garlic and saute until fragrant. Stir in the passata, oregano, and chili flakes. Bring to a simmer and cook until thickened. Season with salt and pepper to taste.
In a separate pan, add remaining oil. Cook peppers and onions until softened, remove and cool.
Roll out pizza dough into a large rectangle. Spread tomato sauce in the center of the dough leaving the edges bare. Top with peppers and onions, sliced sausages, mozzarella slices, and basil.
Roll the dough into a large spiral log from the long end up., pinching the ends to seal tightly.
Transfer to a baking tray. Brush generously with garlic butter. Bake for 30 minutes until golden brown.