Melt 50g/1¾oz butter in a saucepan over a medium heat then stir in the stock. Add the celery and courgette and cook until tender.
To make the dressing, bring a pan of water to the boil, reduce to a simmer, add the egg and cook for 5 minutes until soft boiled.
Meanwhile, mix the oil, vinegar, mustard, garlic and season. Break the yolk of the soft boiled egg into the dressing and whisk. Finely chop the whites and stir into the dressing. Stir in the tarragon and set aside.
Melt the remaining 25g/1oz butter in a pan and sauté the mushrooms and thyme. Add a splash of Madeira and cook until reduced.
In a separate pan, heat dash oil over a high heat and fry the scallop slices on one side until golden; this will take 2–3 minutes. Slice on an angle.
To serve, spoon the mushrooms between two bowls, crisscross the top with the celery and courgettes, spoon over some of the vinaigrette and top with the scallop slices.