1. Blanch the green beans: Bring a large pot of water to a boil. Add the green beans and blanch for 8 minutes. Dump the green beans into a strainer and let sit to allow the excess water drain.
2. For homemade breadcrumbs (8 min prep, 20 min cook): Preheat the oven to 300°F and line a baking sheet with parchment paper. Pulse the slices of bread in a food processor until crumbly. Drizzle the olive oil over the crumbs and sprinkle the italian seasoning, garlic, and salt. Pulse again a few times to incorporate. Spread the crumbs onto the parchment lined baking sheet. Bake 15-20 minutes, tossing halfway, until golden brown and dry.
3. For crispy onions (20 min prep, 15 min cook): Preheat the oven to 450°F and line a large baking sheet with parchment paper. Set aside. Set out 3 small-medium bowls. Add the flour and salt to one bowl and whisk together. Add the egg and milk to a second bowl, whisking them together. Put the breadcrumbs into a third bowl. Slice the onions into about 1/2” slices. Then dip each side of the onion slices into the flour, then the egg mixture, then the breadcrumb mixture. The breadcrumbs won’t coat it completely, but that’s okay! Place onto a baking sheet. Repeat with each onion slice. Bake until golden brown, about 15-18 minutes. Flip them halfway through to brown the opposite side. Once completed, remove from the oven and set aside. Roughly chop up once cooled.
4. Reduce the oven temperature to 400°F.
5. For the casserole (30 mins): Melt 1 Tbsp of butter in a medium skillet over med heat. Add the mushrooms, salt, and the pepper. Cook for about 5 minutes, stirring occasionally. Add the garlic cloves and cook for another minute or two until fragrant and golden. Add another 2 Tbsp of butter as well as 3 Tbsp of flour to start creating a roux: immediately start stirring constantly with a silicone spatula and continue stirring for a couple minutes until the roux is golden. Start using a whisk and slowly pour in the chicken broth and the heavy cream to incorporate. Turn the heat to med-low and let it simmer for about 15 minutes until thickened enough to coat the green beans. If needed, whisk in another 2-3 Tbsp of flour to thicken further. Stir in the grated parmesan.
6. Bake it all together (15 mins): Put the green beans into a 9×13 casserole dish and toss together with the thickened mushroom cream sauce and about 1/4 of the chopped crispy onions. Then top with the remaining chopped crispy onions. Bake at 400ºF for 15 minutes until bubbly. Enjoy!

Notes: 

• If you want the green beans thoroughly coated in the creamy sauce, then use the higher amount of cream in the recipe.

• Make the recipe gluten-free by using gluten-free breadcrumbs and gluten-free flour.

• To prep entire casserole ahead of time: If you want to save yourself the time needed to make this on Thanksgiving, you can prep it up to a couple days before! Keep it tightly covered in the fridge then bake for 20-25 minutes when ready. You can put broil on for just a minute or two to crisp up the onion even more if desired – Just watch it the whole time because it happens fast and you don’t want it to burn. You can also just prep the casserole ingredients but wait until thanksgiving day to put the crispy onions on top of the casserole if you’re wanting the onions to be fully crispy.

• If you don’t want to prep the entire casserole at one time, then you can prep a few parts ahead of time such as the breadcrumbs (store in airtight container in refrigerator for up to 2 weeks), blanched green beans (store in fridge for up to a couple days), and getting the onions sliced and ready to prep (store in fridge).