Turn the oven to 180°C/350°F/Gas 4. Put the sliced onions, extra virgin olive oil, balsamic vinegar, salt and pepper into a ceramic ovenproof dish and seal tightly with foil. Use a dish that’s just large enough to hold the onions. Bake in the oven for 1 hour, remove the foil and cook for another 15 minutes, then remove and cool completely. Drop the temperature to 150°C/300°F/Gas 2 and lay the bread slices on a baking sheet. Brush them with the olive oil and bake until golden on both sides, then remove from the oven to cool.
Cut 2cm (¾in) from the base of each chicory head and separate the leaves, wash and drain. Bring a large pot of salted water to the boil. Holding both ends of each asparagus stem, bend them towards each other until the stem snaps. Discard the lower end (or use in a vegetable stock). Drop the asparagus tips in the boiling water, return to the boil and cook for 1 minute. Drain and refresh in cold water. Mix the onions and their cooking juices, the chicory and asparagus together. Serve on a plate with the croûtons on top.