Preheat your oven to 200°C. Chop the potatoes into 2cm wide wedges (no need to peel). Pop the wedges on a large baking tray. Drizzle with oil, season with salt and pepper then toss to coat. Spread out in a single layer. TIP: Use two baking trays if necessary. Roast on the top shelf of your oven until golden, 25-35 mins. Turn halfway through.

Halve, peel and thinly slice the onion. Trim the root from the baby gem lettuce. Reserve 1 leaf per person, halve the remaining lettuce lengthways, then thinly slice widthways. Heat a drizzle of oil in a large frying pan on medium-high heat. Once hot, add the onion to the pan and cook until softened, 6-8 mins. Stir occasionally. Meanwhile, halve the tomatoes and finely chop the parsley (stalks and all). Grate the cheese. Remove the stalk from the portobello mushrooms (but leave the mushroom whole).

Once the onion has cooked, transfer to a bowl. Return the frying pan to medium-high heat with a drizzle of oil. Add the mushrooms to the pan and season with salt and pepper. Fry until golden brown, 3-4 mins, then turn over and cook for a further 3-4 mins.

Once the potatoes have 10 mins left, move them to the middle of the oven. Transfer the mushrooms (stalk side up) onto a baking tray. Divide the cheddar between the mushrooms then bake on the top shelf of your oven until the cheese has melted, 8-10 mins.

Meanwhile, pop the tomatoes, parsley, sliced baby gem, olive oil (see ingredients for amount) and red wine vinegar into a bowl. Season with salt, pepper and a pinch of sugar (if you have any). Mix together and set aside. Halve the burger buns. Add the BBQ sauce to the onions and stir together.

Pop your burger buns into your oven to warm through for the last 1-2 mins of mushroom cooking time. Divide the buns between your plates and top the bottom half with a whole baby gem leaf and the cheesy mushroom. Spoon over the BBQ onions and top with the bun lid. Serve with the wedges and salad alongside. Enjoy!