Start by preparing the garlic mushrooms. Add 2 tbsp of the olive oil to a frying pan on medium high heat, and sauté the minced garlic for one minute, until fragrant.
Add the sliced mushrooms and sauté to coat in the oil and garlic. Cook for a few minutes, until soft. Season with a pinch of salt and the minced parsley. Reserve.
While cooking the rice in the steps ahead, heat all of the broth you will use in a small saucepan and keep warm until needed.
In a large pot, add the remaining 2 tbsp of olive oil and the 2 tbsp of butter over a medium heat. Once the butter is melted, add the diced onion and sauté for about 7 minutes, until totally translucent and cooked through.
Add the rice to the pot and turn up the heat to medium high. Sauté the rice for one minute to fully coat in the oil and butter and toast it a bit.
Add the sherry wine and let it reduce for about one minute.
Next, lower the heat to medium-low and add the broth about 1/2 cup at a time. Stir frequently and add more broth each time the rice is almost dry.
Keep going until you've added 2/3 of the broth. Then add in the cooked mushrooms and any juices they've released.
Keep adding broth until all is incorporated (though depending on the rice used it may need slightly more or slightly less). The total cook time should match the rice's instructions (usually 17-22 minutes).
When in doubt, taste the rice to make sure it isn't too hard, or starting to overcook.
As soon as it's finished cooking, stir in the grated Manchego cheese. Taste and adjust for salt. Serve immediately with a drizzle of extra virgin olive oil on top if you'd like. Enjoy!