For the most part, I found frozen green beans in 12-ounce packages. If you find 10-ounce packages, I would still stick to 2 packages. I have tried the recipe with 3 (10 ounce) packages and there was not enough sauce for that many beans.
Be sure to use frozen cut green beans, not “French” green beans (the long, stringy ones) or whole, uncut beans (you’d just have extra work cutting them).
To substitute fresh beans, start by trimming them and cutting them in half. Then, bring 4 quarts water and 1 Tablespoon salt to a boil. Meanwhile, fill a large bowl with ice and water. Cook the beans about 5 minutes, until bright green and crisp-tender. Drain the beans and immediately plunge into the ice water. Drain well before adding them to the sauce in Step 5.
To triple the recipe for 30 to 40 people, increase the ingredients as follows: 1/2 cup (1 stick) butter 3 pounds white button mushrooms, wiped clean and sliced or broken into pieces 1 bulb garlic (9 to 10 cloves) 3 teaspoons salt and 1/2 teaspoon pepper 1 cup flour 4 1/2 cups chicken broth 4 1/2 cups heavy cream 6 (12 ounce) packages of frozen cut green beans 3 cups canned fried onions (or more if desired)
3x recipe instruction changes: Cook the mushrooms for about 10 minutes in Step 2 and simmer the sauce for 10 minutes in Step 3. Cook on HIGH for 2 hours or LOW for 4 hours (or until a thermometer reaches at least 165 degrees in the middle of the casserole). Add more canned onions throughout serving time if the top layer is depleted, if desired.
The mushroom sauce may be prepared in advance, cooled, and refrigerated overnight. Increase the crock pot time to 3 hours on HIGH or 5 hours on LOW.
If using fresh beans, they may be blanched the night before, cooled, and stirred into the cooled mushroom sauce and refrigerated overnight. Increase the crock pot time to 3 hours on HIGH or 5 hours on LOW.
Adapted from Cook’s Illustrated Magazine, November & December 2006 issue.