Perfect Green Bean Casserole is the best green bean casserole from scratch.Fresh green beans and mushrooms and crispy fried onions make this a classic!
Rehydrate mushrooms: In a small dish, pour boiling water over the dried mushrooms and allow them to sit for at least 15 minutes. For best results, weigh the mushrooms down with a dish so they remained fully submerged. Do not discard the liquid. Once the pieces are rehydrated and cool to the touch, use a pair of clean sharp kitchen shears to cut them into small pieces. Rehydrated mushrooms (depending on the variety) can be very chewy, so small pieces are better.
Blanch the green beans: Prepare an ice water bath in a large bowl. Add the kosher salt to a large pot of boiling water. Cook the green beans in the salted boiling salted water for about a minute, then immediately transfer them to the ice bath. Once cooled, drain the green beans and set aside. They should be a vibrant green color.
Sauté fresh mushrooms: In a heavy bottom stockpot, melt the butter over medium-high heat and add the sliced mushrooms. Allow them to brown, stirring only to prevent burning, for about 5-8 minutes. Sprinkle the flour over the mushrooms, stir to coat, and allow them to cook for another couple of minutes.
Make the mushroom gravy: Add the chicken broth to the sauteed mushroom roux, stir well, and scrape the bottom of the pan to remove any bits that may have stuck. Add the rehydrated mushroom pieces along with any liquid. Reduce the heat to medium-low, add the milk, stir, and continue to cook until the gravy thickens.
Combine remaining casserole ingredients: Add the blanched green beans and 1/2 cup of the onion pieces to the mushroom gravy. Stir well to combine. Cover with aluminum foil. Either refrigerate overnight or continue cooking.
Cook casserole: Preheat oven to 350°F and bake covered casserole until heated through, about 20-30 minutes. Remove foil and top with remaining fried onion pieces. Continue cooking until they just start to brown. Serve hot.
Notes
Make ahead directions: Prepare the casserole all the way up to the point where it is covered in aluminum foil. Refrigerate overnight. Either allow the casserole to come to room temperature before baking, or place it directly into the oven from the refrigerator but increase cooking time to account for colder temp. Remove the foil and add additional fried onion pieces per recipe instructions.