Preheat the oven to 400 degrees F. Line a large sheet pan with parchment paper or foil for easy cleanup.
Place the sausages, sliced bell peppers, and onion on the sheet pan. Drizzle with olive oil and sprinkle with Italian seasoning, salt, and black pepper. Toss everything to coat evenly, then spread into a single layer. Roast for 25 minutes, turning the sausages halfway through.
While the sheet pan cooks, mix the softened butter with minced garlic and chopped parsley in a small bowl. Slice the French bread loaf lengthwise and spread the garlic butter over the cut sides.
After 25 minutes, remove the sheet pan from the oven. Place the garlic bread on a second baking sheet and add it to the oven. Bake both for another 8 to 10 minutes until the bread is golden and the sausages are fully cooked.
Slice the sausages if desired and serve everything together from the pan.