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1
Preheat oven to 400F.
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2
Cut the leeks (white and very light green portion only) into thin rings. Cut the onion into thin slices. Place both in a small bowl and toss with the olive oil to coat. Add to a baking sheet and add the Panko bread crumbs and salt, toss to combine.
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3
Bake at 400F for about 30 minutes, tossing about every 8-10 minutes. Remove from the oven when a golden brown color is reached.
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4
Place the green beans in a pot of rapidly boiling water. Boil for 5 minutes and immediately drain by pouring the beans into a colander, rinse with cold water for 2 minutes to stop the cooking.
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5
In a sauce pan over medium heat, melt the butter. Add the mushrooms and cook for about 5 minutes, or until they turn a dark brown. Add the garlic and cook for about 30 seconds.
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6
In a separate bowl, add the chicken broth and the flour, stir to combine. Pour into the sauce pan through a mesh strainer to remove any flour lumps. Stir and continue to heat until thickened, about 3-5 minutes. Remove from heat, add the cream and stir until combined. Return to heat and allow to cook until thickened, about 5 minutes. Remove from heat, add the green beans and 1/2 cup of the onion/leek mixture, stir. Pour into a 2 quart baking dish.
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7
*Time management tip: If you want to make this the day before Thanksgiving, this is where you stop. Cover the baking dish and put it into the fridge. Put the remaining cooled onion/leek mixture in a separate container or Ziplock bag to prevent them from getting soggy from sitting on the top of the casserole all night.
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8
When ready to bake: Top your casserole with the remaining onion mixture and place into your preheated 400 degree F oven. Immediately reduce heat to 375F and bake for 15 minutes or until warmed all the way though (note: if you are cooking this after removing from the fridge, as in the above tip, the cooking time will be longer, approximately 22 minutes).
