Get ordinary field mushrooms. I prefer to select each rather than buy in a pack - that way I can get the size I want - it needs to be a size that can fit in the mouth but only just. Also get some sun-dried tomato whether loose or in a jar or whatever. Also get some parsley. Also get some cheese - I use the pre-grated 'pizza cheese' which is a blend of cheddar, mozzarella and parmesan. In a way my stuffed mushrooms in terms of taste are like bite-sized mini-pizzas...
Every mushroom has to have its stalk removed. Sit the mushroom on its cup with the stalk sticking upwards. Grasp the cup firmly in one hand and gently twist and turn the stalk till it comes loose. Hopefully the stalk will have left behind a nice smooth crater in which to stuff. The stalks are a by-product of the process. I suppose you can always mulch them and turn them into part of a pasta sauce.
Now cut the sun-dried tomato pieces to size so that they can be small stipes that then curl into a ring and sit just inside the cup of a mushroom. Pop them in. Now fill in any leftover space in the cup with the grated cheese till it is flush with the top of the cup. This will both keep the tomato inside the mushroom and give the whole thing that wonderful taste and texture that only cheese can bring. Finally sprinkle a pinch of finely chopped parsley on the top of the cheese to both give it extra flavour and also a hint of green to contrast with the yellow and red.
Hmmm... I cannot tell you the timing or the temperature or even whether to use a conventional or microwave oven (I have used both successfully). The important thing is that it cook till the cheese has melted but the mushroom still wants to be moist and full-bodied. In other words it is a pretty short cooking time. Just go with what you know and what works. The things are best served hot as finger food at a party or function. They are vegetarian (as long as the cheese has non-animal rennet) but far from vegan (I did once do these with puréed corn in place of the cheese - that sort of worked...).