"To marinate the mushrooms" means to soak or coat mushrooms in a mixture of liquid and flavors, such as vinegar, oil, herbs, and spices, to enhance their taste and texture before cooking or serving. Full definition
I like to garnish this salad with marinated mushrooms from another recipe in the same cookbook: 1/2 pound of Shiitake and Crimini mushrooms (or any wild mushrooms)-- sliced 3 tablespoons olive oil 3 tablespoons lemon juice some fresh herbs (I used basil and cilantro) some sea salt garnish with freshly ground black pepper (golubkakitchen.com)
Marinate mushrooms for about 30 minutes in a mixture of minced garlic, minced rosemary, balsamic vinegar and olive oil. (leafygreensandme.com)
His hearty vegetarian breakfast sandwich — with its mix of marinated mushrooms, soft eggs, melted cheese, and a buttery toasted roll — proves his point. (bonappetit.com)