Add half 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, or until softened. Season with a pinch of salt and the minced parsley, then reserve.
Pour the broth into a small saucepan and keep it warm until you need it later in the recipe.
Add the remaining olive oil and the butter to a large pot set over medium heat. Once the butter is melted, add the diced onion and sauté for about 7 minutes, until it's totally translucent and cooked through.
Add the rice to the pot and turn up the heat to medium high. Sauté the rice for 1 minute to fully coat it in the oil and butter and to toast it a bit. Add the sherry wine and let it reduce for about 1 minute.
Lower the heat to medium-low and add two-thirds of the warm broth about ½ cup at a time. Stir frequently and add more broth each time the rice is almost dry.
Add in the cooked mushrooms and any juices they've released, then add the remaining one-third of the warm broth. (Depending on the rice used, it may need slightly more or slightly less broth.)
Cook until the rice has a good texture: not too soft or hard, and not overcooked. The total cook time should match the rice's instructions (usually 17-22 minutes).
As soon as it's finished cooking, stir in the grated Manchego cheese and season with salt to taste.
Serve immediately with a drizzle of extra virgin olive oil on top if you'd like. Enjoy!