Clean mushrooms to remove all the dirt. If you are cutting mushrooms in advance, then make sure to dry them well with a paper towel before cutting, else the mushrooms will turn black.
Cut mushrooms into thick slices.
In a wide dutch oven or a skillet deep enough, add a tablespoon of oil. when hot, add half of the mushroom (make sure not to overcrowd the pan) . Cook for 5-6 minutes undisturbed. lightly season with salt and pepper. Flip and cook for another 5 minutes until mushroom is nicely browned.
Remove the browned mushroom, keep aside. Add remaining oil to the pan. Repeat the above process with remaining mushrooms. Keep aside.
In the same pan melt butter, add chopped onion and garlic. Saute for 2-3 minutes until it is aromatic.
Add rice, thyme and pepper. Mix it and toss the well so that rice gets coated with butter. Toss rice for 2-3 minutes. We don't have to brown the rice here. ( ref note 3).
Add warm vegetable stock/ water. Add salt per taste. (ref note 4). When it comes to a boil, lower(simmer) the heat, cover and cook for 5 minutes.
After 5 minutes, add cooked mushroom. Cover and cook for another 10 minutes on low heat until there is no residual liquid in the pot. (ref note 5)
Remove the lid, sdd chopped parsley. Gently mix rice and mushroom. Cover and let it rest for 10 minutes.
Fluff the rice and serve.