First of all chop the mushrooms into roughly 1 inch (2.5 cm) chunks - it's going to look an enormous quantity at this stage, but they will lose approximately half their volume in the initial cooking. Now heat a teaspoon of the olive oil in a frying pan and, when it's hot, throw in the mushrooms and toss them around by shaking the pan. Don't worry that there is so little oil, because the mushrooms give off masses of juice once the heat gets to them. Season with salt and pepper, then turn the heat down to very low and just let the mushrooms cook gently, uncovered, so that all the juice evaporates and the flavour of the mushrooms becomes more concentrated. Leave them like that for 30 minutes, stirring them around once or twice.
While they are cooking, cut two-thirds of the cheese into ¼ inch (5mm) cubes and grate the other third on the coarse blade of the grater. After that, break the eggs into a large bowl, whisk lightly with a fork and season well with salt and pepper. Then add three-quarters of the cooked mushrooms to the eggs, together with the cubed cheese. Place the rest of the mushrooms in a bowl covered with foil and keep them warm in the oven, along with the plates.
Now wipe the pan clean with some kitchen paper and put it back on a medium heat, add the rest of the olive oil and, when it's hot, swirl it around the pan. Turn the heat down to its lowest setting and pour the egg mixture into the pan, scattering the grated cheese all over the surface. Now all you have to do is leave it alone and put a timer on forminutes.
When 15 minutes have passed, turn the grill on to its highest setting and see how the omelette is cooking - it will probably take about 20 minutes in total to cook, but there should still be about 10 per cent of liquid egg left on the top. At that stage transfer the pan to the grill - not too close - and cook briefly to allow the liquid egg to set. This will take 20-30 seconds. Scatter the remaining cooked mushrooms over the top of the frittata and cut it into four wedges. Transfer the wedges to warm plates and serve immediately, because the egg will continue cooking even though the frittata is no longer in contact with the heat. I like to serve this with two salads - a plain, green lettuce salad and a tomato and basil salad.