Chop the onion finely. Peel the zucchini and cut them into medium-sized bites along with the mushrooms (they shrink a lot later - especially the mushrooms). Dice the pepper. Wash the rice.
Heat oil in a tray, braise the onion, pepper and the mushrooms in it. Then put a lid on and leave them at a lower heat until the mushrooms soften a bit, while stirring from time to time so they don't stick.
Once they soften, add the zucchini. Leave them to soften too, then add the rice. Pour in about 6 cups water, turn the stove up until the water comes to a boil, then lower again to a simmer.
Once the rice has absorbed most of the water, add the tomato paste and salt to taste. Leave to simmer until ready.
If the rice ends up a little too watery, it will thicken a bit by the next day or a little later.