For the mushrooms and shallots, preheat the oven to 220C/200C Fan/Gas 7.
Heat 50g/1¾oz of the butter and the oil in a small, ovenproof frying pan over a medium heat. Add the shallots, then transfer the pan to the oven and roast the shallots for 15 minutes, or until tender.
Remove the roasted shallots from the pan and set aside. Once cool enough to handle, separate the shallots into 'petals' by peeling each whole layer.
Place the same pan over a medium heat, then add the remaining butter, the mushrooms and the garlic and fry for 3-4 minutes, or until softened and starting to brown.
For the baby leeks, blanch the baby leeks in a large saucepan of boiling, salted water for 1-2 minutes. Drain and refresh in a bowl of ice-cold water.
Heat a griddle pan over a high heat. Rub the leeks all over with a little oil. Add the leeks to the griddle pan and chargrill for 2-3 minutes on all sides, or until golden-brown chargrill marks appear.
Keep all of the vegetables warm until ready to serve.
For the soufflés, preheat the oven to 180C/160C Fan/Gas 4. Grease the base and sides of 4 ramekins with a little butter, then dust with flour, shaking out any excess. Place the ramekins into a deep roasting tray.
Heat 50g/1¾oz of the butter in a frying pan over a medium heat. Add the mushrooms and fry for 1-2 minutes, or until softened. Season, to taste, with salt and freshly ground black pepper.
Melt the remaining butter in a large saucepan over a medium heat. Whisk in the flour to form a thick paste (known as a roux). Cook the roux over a low heat for a further 2-3 minutes, stirring with a wooden spoon.
Gradually whisk in the milk, a little at a time, whisking constantly to prevent any lumps from forming. Once all of the milk has been incorporated into the roux, reduce the heat to its lowest setting and continue to cook for a further 2-3 minutes, or until the mixture is thick enough to coat the back of a spoon.
Stir in 125g/4½oz of the cheese, as well as the mustard and cooked mushrooms. Beat in the egg yolks and season again with salt and pepper, to taste.
In a clean, dry bowl, whisk the egg whites until stiff peaks form when the whisk is removed. Fold the egg whites carefully into the soufflé mixture.
Divide the mixture equally among the prepared ramekins. Pour enough boiling water into the roasting tray to reach halfway up the sides of the ramekins. Carefully transfer the roasting tray to the oven and bake the soufflés for 15-20 minutes, or until they have risen and are evenly browned.
Remove the soufflés from the roasting tray and set aside on a heatproof surface for 5 minutes.
Preheat the grill to its highest setting.
Run a round-bladed knife around the inner edges of the ramekins, then turn out each soufflé into an individual ovenproof dish.
Pour one-quarter of the cream into each dish, coating the top of each soufflé. Sprinkle over the thyme and the remaining cheddar, then grill the soufflés for 3-4 minutes, or until the cheese is bubbling and golden-brown.
To serve, place the soufflé dishes onto serving plates. Spoon the mushrooms, shallots and baby leeks alongside.