Champiñones Gratinados: Mushroom Gratin with Ham Mayonnaise

This easy mushroom gratin recipe is simple, rich and full of flavour. Made with meaty portobello mushrooms, fresh thyme, spring onion and a creamy serrano ham mayonnaise, it is a delicious tapa, starter or small plate to serve when you are cooking for friends or family. The mushrooms roast gently until tender, then they are finished under a savoury topping that turns lightly golden in the oven.

Champiñones gratinados are ideal when you want something impressive without making the preparation complicated. The ingredients are straightforward, the cooking time is short, and much of the work can be done just before serving. The mushrooms can sit warm in the oven for a short time, which makes this recipe especially useful when entertaining. Serve them as part of a tapas spread, as a first course before a main meal, or alongside bread to enjoy every bit of the creamy topping and roasting juices.

The key to this dish is balance. Portobello mushrooms bring a deep, earthy flavour, while thyme adds freshness and aroma. The spring onion gives a gentle sharpness, and the ham mayonnaise adds saltiness, richness and a smooth texture. Use mild olive oil for the mayonnaise so it does not overpower the serrano ham. Season carefully, as the ham already brings salt to the dish.

Ingredients

For the mushrooms

• 4 portobello mushrooms, stalks trimmed

• 2 sprigs fresh thyme, leaves separated

• Olive oil, for drizzling

• The green part of 1 spring onion, finely chopped

• Sea salt and freshly ground black pepper, to taste

For the ham mayonnaise

• 1 egg yolk

• 1 garlic clove

• A pinch of salt

• 1 tablespoon lemon juice or vinegar

• 3 slices serrano ham, roughly chopped

• 150 ml mild olive oil

Preparation

1. Place the portobello mushrooms on a roasting tray with the open side facing up. Sprinkle them with a little sea salt, freshly ground black pepper and the fresh thyme leaves. Drizzle lightly with olive oil, making sure the mushrooms are coated but not swimming in oil. Put the tray into a cold oven, then set the temperature to 180C/350F/gas mark 4. Bake for about 15 minutes, or until the mushrooms have softened and released some of their juices.

2. While the mushrooms are roasting, prepare the ham mayonnaise. Add the egg yolk, garlic clove, pinch of salt, lemon juice or vinegar, and chopped serrano ham to a food processor. Blend until the mixture begins to come together. With the processor running, slowly pour in the mild olive oil in a steady stream. Continue blending until the mixture thickens into a smooth mayonnaise. Add the oil gradually, as this helps the mayonnaise emulsify properly and gives it a creamy texture.

3. Remove the roasting tray from the oven and sprinkle the mushrooms with the finely chopped green spring onion. Spoon a generous dollop of the ham mayonnaise over each mushroom. Return the tray to the oven and bake for another 5 minutes, or until the topping is warmed through and lightly browned on top. Keep an eye on the mushrooms during this final stage so the mayonnaise colours nicely without overcooking.

4. Serve the mushroom gratin hot, straight from the oven. If you are preparing several dishes at once, you can keep the mushrooms warm in the oven for a short time until you are ready to serve. They are best enjoyed when the mushrooms are tender, the topping is golden, and the flavours are still fresh and aromatic.

Serving Suggestions

These champiñones gratinados work beautifully as a tapa or starter. Serve one large portobello mushroom per person as an appetizer, or place them in the centre of the table as part of a relaxed sharing meal. They go well with crusty bread, which is perfect for soaking up the savoury juices from the roasted mushrooms and the creamy ham mayonnaise.

For the best result, use mushrooms that are firm and evenly sized so they cook at the same pace. Trim the stalks neatly, but do not remove too much of the mushroom, as the shape helps hold the topping. Fresh thyme is recommended because it gives a bright herbal flavour that suits the richness of the mayonnaise. If the serrano ham is very salty, reduce the extra salt slightly and adjust the seasoning only after tasting.

This mushroom gratin recipe is quick, satisfying and full of character. With just a handful of ingredients and a short time in the oven, it turns simple portobello mushrooms into a warm, flavourful dish that feels special enough for guests while still being easy enough to make at home.