Heat the oil in a pan. Sauté the squash and onions for approx. 5 mins., season. Add the apple juice and briefly bring to the boil.
Arrange the slices of bread on a baking tray lined with baking paper, drizzle with the juice and brush with mustard. Scatter the cheese and squash over the top.
Bake for approx. 15 mins. in the upper half of an oven preheated to 200°C. Sprinkle with the reserved onion greens.