As we mentioned earlier, you should make the besamel about 24 hours in advance. Wash your mushrooms carefully with water and chop them into small pieces of about 1 or 2 cm.
Heat a tablespoon of olive oil in a skillet over medium heat. Add the mushrooms, a pinch of salt and stir-fry for about 2 minutes.
Place them in a blender jar and blend until you get a creamy texture.
Chop the spring onion into small pieces. Heat the butter in a skillet over low heat. Once the butter has melted you can add the spring onion and fry for 4 minutes.
Add the flour and cook it for about 30 seconds more, stirring continuously with a wooden spoon if possible and make sure the flour doesn’t burn. Then, add a bit of milk and keep stirring.
Add the blended mushrooms. Keep adding milk little by little while stirring until you get a homogenous and not too liquid but not too solid sauce. It may take 15-20 minutes. If you feel like the sauce is too liquid just keep stirring a bit more over low heat. If it's too solid, add some more milk.
Place your besamel in a plate, covered with another plate or with some plastic wrap. Keep it in the fridge once it´s cold.
The next day, scoop up some of your besamel using a spoon and create an oval shape with your hands. Repeat the process until you have run out of besamel.
Bread your croquetas in two phases. Place a whisked egg on one dish, and breadcrumbs on the other. Take a croqueta, soak it in the egg, roll it on the bread crumbs and repeat the process with all your croquetas.
Heat about 3 centimeters of olive oil in a big pan over high heat and start frying the croquetas. To make sure the oil is hot enough, put one of the croquetas in the pan: if you see some bubbles appearing it means the oil is ready. When you are frying them, move them around on your frying pan to ensure even browning, and to ensure they don’t burn. Turn them over and repeat on the other side. Then remove them and place them on a plate lined with kitchen towels to soak up the oil.