If using dried shiitake mushrooms, soak them in warm water until softened. Gently squeeze out the excess liquid, then slice. Strain the mushroom soaking liquid through a fine-mesh sieve and reserve it for cooking the rice.
Heat a large, deep pot over medium heat. Add the neutral oil, onion, and garlic. Sauté for about 2 minutes, until fragrant and the onion begins to turn translucent.
Increase the heat to medium-high. Add the oyster mushrooms, shimeji mushrooms, and shiitake mushrooms. Cook until softened and most of the excess moisture has evaporated.
Add the drained rice, soy sauce, salt, and black pepper. Gently stir to combine, then spread the rice into an even layer.
Pour in 1 ⅔ cups total liquid for soaked rice, using the reserved shiitake soaking liquid as part of the amount and topping up with water as needed.
Scatter the diced Chinese sausage evenly over the top and cover with a tight-fitting lid.
Once the liquid begins to bubble, immediately reduce the heat to low. Cook, covered, for about 12 minutes.
If using unsoaked rice, use 2–2 ¼ cups total liquid and cook for 15–18 minutes instead.
Turn off the heat and let the rice rest covered for 10 minutes. Avoid opening the lid during cooking or resting.
Remove the lid and add the butter while the rice is still hot. Garnish with chopped green onions, gently mix everything together, and serve warm.