Heat olive oil in a pan over medium heat. Cook sausage links until browned and oil begins to render, about 3 minutes per side. While sausages are cooking, pierce them lightly here and there with the tip of a sharp knife so some fats and juices are released. Remove from heat and let cool slightly.
When sausages are cool enough to handle, cut them into serving pieces, about 5cm slices. Transfer back to pan along with any accumulated juices from the cutting board.
Cut two slashes down to the bone on the skin side of each chicken thigh.
Depending on the size of the peppers, halve or quarter them and place in a large mixing bowl. Add the sliced red and yellow onions and potato chunks. Add chicken thighs and sausage pieces with pan juices.
Season with coarse sea salt, black pepper and Italian herbs. Drizzle with a tablespoon of olive oil.
Mix with your hands until all ingredients are coated in oil, 3 or 4 minutes. Transfer to large, heavy-based roasting pan. Evenly space the chicken thighs skin side up. Position potatoes near the top.
Place in preheated oven until chicken is cooked through and everything is caramelised, about 1 hour. Sprinkle with chopped fresh Italian parsley, if desired.