Preheat oven to 180C/350F. Place mushrooms in a large bowl. Pour over hot melted butter, add garlic, salt and pepper. Toss well. Place all the Rice ingredients in a baking pan - 22 x 33 cm / 9 x 13". Mine is 30 x 24 cm / 8 x 9.5" - don't need to be exact here, but don't stray too much from pan size. Mix and shake the pan to spread the rice out evenly. Spread the mushrooms over the rice - they will be partially submerged but should pretty much cover the whole surface area (to be the "lid" while the rice cooks). Drizzle over olive oil. Bake for 35 - 40 minutes or until mushroom is golden. Remove from oven and rest for 5 minutes. Finishes: Add extra butter (if using) and scallions. Toss through rice. Serve!