Vegetarian recipes: Julia Platt Leonard's king oyster mushroom & thyme tart

King oyster mushrooms could make the most committed carnivore decide to go vegetarian

Julia’s tart is topped with king oyster mushrooms, caramelised onions, goat's cheese and fresh thyme

Of course I make a list before I go grocery shopping but that doesn’t mean I follow it. Take for example a visit to the green grocer. I’ve got what I call my ‘don’t you dare forget to pick-up…’ things like lemons and garlic and berries for breakfast. But I come home with a lot of food that wasn’t on my list – things that catch my eye or call out to my stomach.

Take the king oyster mushrooms I found the other day. I don’t see them that often so when I do, I snap them up. King oyster – also known as king trumpet – mushrooms are the largest members of the oyster mushroom genus. While traditional oyster mushrooms are smaller with an almost frilly cap, king oysters – as the name implies – are larger, around 11-12cm long, with a thick white stem and tight tan cap.

They’re meaty, rich and packed full of flavour. King oysters are also firm enough so you can easily slice them into ‘steaks’ and roast them in the oven. They’re the kind of mushroom that could make the most committed carnivore decide to go vegetarian.

For something so majestic, I find that less is truly more. I’d happily slap some roasted slices on a piece of sour dough bread with a bit of taleggio cheese and call it a day. For something a bit more refined, I make this tart with the mushrooms, caramelised onions and a bit of fresh thyme. With the help of ready rolled, all butter pastry, it’s a quick and easy dish that only needs a salad to call itself a meal.

If you can’t find king oyster mushrooms? The tart would be happy with a mix of brown button, shitake and the more diminutive (but easier to find) oyster mushrooms. But keep your list in your hand and eye watchful – you never know when you’ll see the King.

King oyster mushroom with caramelised onion & thyme tart

Serves 4-6

I use frozen all-butter puff pastry that comes already rolled out. All you need to do is let it thaw and you’re good to go.

Preheat oven to 225°

4 large king oyster mushrooms, around 280g total
90ml olive oil (around 6 Tbsp)
2 small-medium red onions
1 tbsp red wine vinegar
A few sprigs of fresh thyme
30g goat’s cheese
375g puff pastry 

Place the defrosted pastry on a baking sheet and place in the refrigerator until you’re ready to use.

Slice the mushrooms length ways into approximately 1/2cm thick slices. Place them on a baking sheet. Brush both sides with olive oil – you’ll need about 60ml for this – and sprinkle with a bit of salt. Reserve the rest of the oil to use with the onions. Place the pan in the hot oven and cook for about 7 minutes then turn the mushrooms over and cook for another 7 until both sides are golden brown. Remove from the oven, place the mushrooms on a plate and allow to cool.

Serve the tart hot or at room temperature, with salad

While the mushrooms are cooking, peel and slice the onions in half and then slice thinly. Place the sliced onions in a medium-sized saucepan with the remaining olive oil and a healthy sprinkle of salt. Cover with the lid and cook over medium heat. Stir occasionally and continue cooking until the onions turn darker in colour and have a jammy consistency. They’ll begin to stick a bit to the bottom. Add the red wine vinegar and scrape the pan to loosen all the bits. Remove from the heat and place the onions in a bowl and allow to cool.

Take the pastry out of the refrigerator and top with the mushrooms and onions. Sprinkle some thyme leaves on top and dot with the goat’s cheese. Place into the hot oven and bake for around 10 minutes or until the pastry is nicely browned and crisp. Serve the tart hot or at room temperature.