Heat the oil and sauté the onion in a large heavy-based saucepan until onions are soft and golden. Add the mushrooms, season with salt and pepper and fry for 5 minutes until golden and juicy.
Heat 3 cups water 2-quart saucepan over medium heat until it comes to a simmer. Reduce heat to low and keep warm. Meanwhile, melt 2 tbsp butter in large saucepan over medium heat; add 1/2 onion and cook until soft. Add rice and stir for 2 to 3 minutes. Add 1/3 water; stir until absorbed. Increase heat to medium-high; stir in 1 cup water-broth mixture. Cook uncovered, stirring frequently, until liquid is absorbed. Continue stirring and adding remaining water mixture, 1 cup at a time, allowing each cup to be absorbed before adding another. Cook for approximately 25 to 30 minutes or until rice is tender and mixture has a creamy consistency.
Add spinach and heat for 1 minute, stirring. Serve immediately with cheese on top