1. Chop the mushrooms into small pieces. Mince the garlic. Begin preheating the oven to 350ºF.

2. Heat a large skillet over medium, then add the olive oil. Swirl to coat the surface of the skillet, then add the mushrooms, garlic, and a pinch of salt and pepper. Sauté the mushrooms until they release all their moisture, the skillet dries up again, and the edges of the mushrooms begin to brown slightly (7-10 minutes).

3. Add the margarine and flour the skillet with the mushrooms. Continue to sauté for 2-3 minutes, or just until the flour begins to coat the bottom of the skillet and turns golden brown.

4. Whisk the vegetable broth into the skillet until all of the flour has dissolved off the bottom of the skillet. Allow the broth to come to a simmer as you whisk, at which point it will thicken.

5. When the liquid in the skillet has thickened, whisk in the milk. Allow the mixture to return to a simmer after adding the milk. Taste the sauce and add another pinch of salt and pepper if needed. You want the sauce to be slightly over seasoned so the flavor doesn't get too diluted once the green beans are added.

6. Remove the skillet from the heat and stir in the green beans (no need to thaw). The cold beans will thicken the sauce considerably. Stir in half the fried onions. Transfer the beans and sauce to a 2-3 quart casserole dish. Place the oven.

7. Bake the Green Bean Casserole for 20 minutes. Top with the remaining fried onions, and bake for another 10 minutes. Serve warm.