In a sauté pan on low heat, cook the frozen spinach until it is completely loosened up and warm. Then drain it and set it aside. Bring the broth to simmer.
Meanwhile, add the mushrooms in a dry frying pan over high heat and cook them for 5 minutes. Then turn the heat to low/medium and cook them for another 15 minutes. When all the liquid is evaporated, add a tablespoon of butter.
Clean the sauté pan and put it on low heat again, add oil and yellow onion. Cook until it is soft and translucent. (5min)
Now add rice and toast it for a minute or two. Then add a ladle of broth and let it cook until it is all absorbed, then add a new spoon. Keep repeating this process till the rice is almost al dente. (14-17 minutes) Stir often.
When the rice is almost done, stir in spinach.
Taste the rice. Is it al dente? If yes, turn off the heat. Now add Parmesan cheese and butter. Stir it and let it rest for a minute.
Give it one last stir, spoon into plates and top off with the sauteed mushrooms.