Chop mushrooms very very finely. Heat butter/oil mix (I use 2 to 1 butter/oil ratio) over medium heat in large skillet/saute pan. When butter/oil bubbles flatten in the pan, saute shallots (and some garlic, not a lot, if you wish) in medium pan until translucent.
Quantities depend on how many mushrooms you have and how much you like shallot flavor.
Increase heat to mediuim high, add finely chopped mushrooms. In a little while, mushrooms will begin to give off liquid and become aromatic. Continue with saute until mushroom liquor is evaporated/reabsorbed. Lower heat if they start to brown or browning aroma is smelled. Mushrooms will condense into a mash. Stop saute when no more liquid is left and mix is rather dry.
Cool and freeze in ice cube trays. I do not add salt as I found it increases mushroom liquids which I am only evaporating. Since I do not add salt, I do not add pepper, lest I become confused. I know S&P is needed when they are recooked.
Makes excellent stuffing alone or as ingredient for almost anything short of jelly donuts. Hmmm, maybe small unsweeted raised donuts duxelle as an appetizer???
Sorry for the rambling recipe. This is how I cook and I encourage those who ask for advice to cook the same way. IMHO, except for baked goods, highly structured recipes are the worst way for people to learn how to cook or improve their cooking.
good luck.