Heat olive oil in a saucepan over medium heat.

Add the chopped onions and cook until they are softened and translucent, about 3-4 minutes.

Add the diced mushrooms to the saucepan and cook, stirring occasionally, for about 3-4 minutes until they release their moisture and start to brown.

Stir in the shelled pistachio nuts and basmati rice, coating them with the oil and mushroom mixture.

Pour in the water or vegetable broth and season with salt and pepper to taste.

Stir well to combine all ingredients.

Bring the mixture to a boil, then reduce the heat to low.

Cover the saucepan with a lid and let it simmer for 15 minutes, or until the rice is cooked and all the liquid is absorbed.

Once the rice is cooked, fluff it with a fork and taste for seasoning.

Adjust salt and pepper if needed.

Garnish with fresh parsley if desired, then serve hot as a delightful side dish to accompany your main course.