I don't often talk about cooking in English, if you find any errors, please email me at firstname.lastname@example.org. This is an open source recipe!
Add flour (but save a small amount), yeast, few tablespoons of olive oil and some salt to the bowl, and mix it for a bit. Then slowly add 250 grams of lukewarm water, while you are mixing. Continue mixing until it is a uniform lump of dough. **Only** if your dough does not get contiguous, add a very small amount of water, and mix some more.
Now sprinkle the dough with some of the remaining flour, turn off the oven, put something over the bowl, and put it in the oven. This needs to rise for one hour, although shorter periods appear to work too.
While this is rising, cut the mushrooms (if you didn't buy presliced ones), and fry them in the olive oil. Now peel the onion (peel a lot, they're cheap), and cut it to tiny chunks. The mixer-with-slicer addon might help, but you can do this yourself.
Don't slice it to slush though, it needs to remain oniony. Add to the mushrooms, and fry for a bit. Should not get brown, just hot. Taste the mushrooms to make sure they are done, then add the tomato sauce, and turn off the gas.
This is a good point to turn your oven on again, set it to 220 degrees Celcius.
Flatten out one part of the dough using a rolling-pin, or something improvised. How thin you make is your choice. I personally prefer a thin pizza, but make sure that you can lift it to put it on the oven plate! If that doesn't work, just refold your dough and flatten it again. You can do this at length until you are happy.
Oil the plate (you don't have to use the expensive olive oil, btw). Put the dough on the plate, and start spreading your mushroom-onion-tomato-sauce mixture on it, not too thick as this will cause your pizza to go soggy. Congratulations, you now have a basic pizza!
Insert into your oven, and monitor carefully. There is no firm rule for when your pizza is ready. Just make sure that the cheese has melted and that the dough has become firm. Check often.
Hint:Make one pizza with a tried and trusted recipe, and experiment with the other!