Preheat the oven to 180°C.
Fry the onion in butter and oil for 18-20 minutes until golden, then season.
Boil water and sugar for 3-4 minutes to form a golden-brown caramel.
Pour a quarter of the caramel into the bottom of four small ovenproof pans.
Roll out puff pastry on a floured surface and cut four 12.5cm discs.
Layer smoked duck slices over the caramel and top with the fried onions.
Brush pan rims with beaten egg, place pastry discs on top, and press to seal.
Bake for 18-20 minutes until the pastry is risen and golden.
Whisk mustard, lemon juice, and olive oil together, then season for the dressing.
Invert each tatin onto a plate and serve with microleaf salad, dressing, and plum chutney.