- Preheat the oven to 180°C/gas 4.
- Add the onion to a pan with the oil and cook until well browned.
- Remove the stalk from the mushroom, chop it up and add it to the onion, along with the spinach, thyme and garlic.
- Once the spinach has wilted, add the mustard, bread and pepper, and mix well. Cover and leave for a couple of minutes so the bread absorbs all the flavours.
- Mix the stuffing again and then pile it onto the mushroom cap.
- Sprinkle the cheese evenly on top, pressing it into the stuffing, and bake for 10 minutes.
- Frozen spinach is a useful ingredient to keep in your freezer. Look out for leaf spinach, rather than chopped, and always defrost it before using so that you can squeeze out the excess water.
- Fresh spinach leaves are a great alternative to lettuce.
- If you don't like Wensleydale, try adding the same amount of Cheddar, feta or goat's cheese.
- Freezing instructions: Freeze with greaseproof paper between each mushroom. Defrost for 1 hour, then heat in the microwave or a moderate oven.