For the mushrooms:

1. In a skillet, heat oil and butter over high heat.
2. Add all mushrooms and cook for a few minutes without stirring, until they start to brown a little. Do this over high heat and without salt so the mushrooms brown before they start releasing their water.
3. Add a little salt and pepper and cook for 1 more minute, stirring a bit. Reserve.

For the risotto:

1. Bring broth to steady, slow simmer in a saucepan.
2. Heat butter and oil in a heavy pan over moderate heat. Add scallions, lightly salt, and sauté for 1 or 2 minutes, until beginning to soften, being careful not to brown them.
3. Add the rice with the thyme and, using a wooden spoon, stir for 1 minute, making sure the grains are very well coated. Add the sherry or white wine, stir once and let it be until it´s almost absorbed.
4. Add ½ cup of broth and half of the mushrooms. Stir well and cook over medium/low heat. Then continue adding the simmering broth, ½ cup at a time, after the last is almost absorbed. Stir frequently, so that the rice releases the starch and it all turns into a creamy mixture.
5. The risotto will take about 18 to 20 minutes, the rice will be tender but still firm. Remove the pan from the heat and immediately add the reserved 4 Tbs butter, asiago cheese, and parsley. Stir vigorously to combine with the rice. Add black pepper and check for salt, and add more if needed.
6. Transfer to a serving platter, mound the remaining mushrooms on top, chopped scallions, a drizzle of olive oil and serve immediately. Risotto waits for no one.