Lighter option, ready in 20 minutes. White sausage works well in many configurations.
Thinly slice the sausage diagonally, then fry the slices in a hot frying pan with the addition of olive oil. Cut the buns or bagels in halves. When the sausage is browned, remove it from the pan and brown the cut rolls on the remaining fat.
Slice the tomatoes. Thinly slice or julienne the onion. Brush the browned bread with pesto. Arrange the slices of coloured tomatoes and the slices of red onion on the bread. Place the sausage slices on top. Sprinkle it all with the chopped green onions and close the sandwich.