Soak the mushrooms in boiling hot water for about 10 minutes. Strain and set the soaking water aside. Discard the mushrooms.
In a saucepan, gently brown the onion and garlic in the oil. Season with salt and pepper. Add the reserved water and rice. Bring to a boil. Cover and cook over low heat for about 20 minutes or until the rice is tender.
Djon djon mushrooms are grown in Haiti. They are used to flavor and to color djon djon rice, a classic of Haitian cuisine. There are djon djon mushrooms in Creole grocery stores.