Preheat oven to 200°C/400°F.

Heat an ovenproof 12in/30cm round heavy based pan over medium heat. Add the butter and let it melt, then put in the mushrooms, leeks, thyme and garlic and increase the heat to medium high. Cook, stirring occasionally, until the mushrooms have softened and shrunk a little. Add the sherry, salt and pepper, and continue cooking until the leeks are quite soft and the mushrooms have cooked down to about a third of their volume. It’s fine if there’s a little liquid in the pan, but the vegetables shouldn’t be swimming. If the leeks are starting to brown before the mushrooms are cooked, reduce the heat slightly to stop them burning.

Remove the pan from heat and lay the pastry directly over the top of the mushroom mixture to completely cover it. I trim the corners of the pastry to fit the pan, though you could just tuck them in a bit if you aren’t worried about presentation.

Put the pan in the oven and cook until the pastry is puffed and golden, around 20-25 minutes. Remove from the oven and immediately invert onto a large plate or a board for serving. Scoop up any vegetables which get left behind in the pan and put them back onto the tart.

Slice into pieces and top each piece with a scoop of mascarpone cheese to serve (the mascarpone isn’t essential but it adds a lovely richness). Serve hot or warm with a vinegar and mustard-dressed green salad alongside.

Preheat oven to 200°C/400°F, combination steam setting. If your oven has variable steam, select 50% (if you don’t have variable steam, just select combination or convection steam setting at the correct temperature, and the oven will work out the humidity for you).

Put the butter, mushrooms, leeks, thyme and garlic into a 12in/30cm round heavy based pan, or a 9x13in/23x33cm rectangle pan. Give it a quick stir but don’t worry too much about everything being mixed perfectly.

Put the pan into the preheated oven and cook for 5 minutes, then stir to mix the now-melted butter through. Add the sherry, salt and pepper, mix briefly again, then return the pan to the oven for a further 8-10 minutes, or until the leeks are quite soft and the mushrooms have cooked down to about a third of their volume. It’s fine if there’s a little liquid in the pan, but the vegetables shouldn’t be swimming.

Remove the pan from the oven and lay the pastry directly over the top of the mushroom mixture to completely cover it. If you’re using a round pan, you can either trim the corners of the pastry to fit the pan, or just tuck them in a bit if you aren’t worried about presentation. In a rectangle pan you’ll have to push the mushroom mixture towards the center of the pan to make a square mound of vegetables that’ll fit under your pastry sheet.

Return the pan to the oven and cook until the pastry is puffed and golden, around 12-15 minutes. Remove from the oven and immediately invert onto a large plate or a board for serving. Scoop up any vegetables which get left behind in the pan and put them back onto the tart.

Slice into pieces and top each piece with a scoop of mascarpone cheese to serve (the mascarpone isn’t essential but it adds a lovely richness). Serve hot or warm with a vinegar and mustard-dressed green salad alongside.