I've mentioned in several recipes that white button mushrooms will darken in color when cooked. Yet, I've had a few occasions where they remained a light cream color, much like store-bought canned mushrooms. Those have been boiled in liquid over low heat, rather than sautéed or stewed. This works best with young mushrooms that have not opened their cap yet.
The mushrooms in the 1st recipe were boiled in orange juice. The acidity of the juice contributes to lightening the mushroom color. The same could be obtained by adding lemon/lime juice or vinegar.
This would be a good method for cooking the mushrooms for vol-au-vent. Just remember, the more liquid you use, the more flavor you'll lose, unless you incorporate that liquid back into your dish.
Pic1-3: mushrooms braised in orange juice and beef roast drippings.
2020-05-29Pic4-6: boiling mushrooms