To make the tarts, melt half the butter with a tablespoon of the oil in a frying pan and fry the mushrooms over a high heat for 2 minutes, or until lightly browned, stirring regularly. Add the remaining oil, onion and leek, reduce the heat slightly and cook for 5 minutes, or until softened, stirring regularly. Add the garlic, season with salt and lots of ground black pepper and cook for a minute more, stirring. Tip onto a plate and spread out so the vegetables can cool a little.
Return the pan to the heat, stir in the remaining butter and as soon as it has melted, stir in the flour. Slowly add the stock, stirring all the time and bring to a simmer. Stir in the cream and simmer gently for 1–2 minutes, or until the sauce is smooth and has thickened; season to taste then set aside.
Unroll the puff pastry, cut into four even rectangles and put on a baking tray lined with non-stick baking paper (or the paper the pastry came with).
Place a spoonful of cranberry sauce in the centre of each rectangle of pastry. Spoon the mushroom mixture on top of the cranberry sauce. Pull in the sides of the pastry, so it half covers the vegetables. Place a small piece of non-stick baking paper on top of the vegetables to prevent them over-browning. Set aside.
Preheat the oven to 220C/200C Fan/Gas 7. Put the potatoes in a large saucepan, cover with water and a lid. Place over a high heat and bring to the boil. Reduce the heat and simmer for 5 minutes.
Drain the potatoes in a colander and let stand for 5 minutes, then return to the saucepan and shake vigorously to knock the potatoes about and scuff up the surface – this will make them much crisper when they roast. Add the oil, season well with salt and pepper and toss together. Scatter the potatoes over a large baking tray. Roast the potatoes for 15 minutes.
Take the tray out of the oven, turn the potatoes and add the carrots. Return to the oven for a further 15 minutes.
Take the tray out of the oven and turn the vegetables. Return to the oven. Place the tray with the tarts on a shelf above the roasting potatoes and carrots. Bake together for 25–30 minutes, or until the pastry is puffed up and golden brown. Turn off the oven and leave the door slightly ajar while the other vegetables are cooked.
Put the Brussels sprouts into a microwaveable serving dish, add 3 tablespoons water, cover and cook on full powder for 4–5 minutes, or until tender. Toss with the butter, salt and ground black pepper.
Put the peas in a microwaveable serving dish, cover and microwave on full powder for 4–5 minutes, or until hot. (You can boil the sprouts and peas if you prefer.)
Warm the cream sauce until bubbling, stirring constantly and adding a splash of water to loosen if necessary, then transfer to warmed jug. Serve the tarts with the roasted vegetables, sprouts, peas and cream sauce.