Prepare an ice bath. In a large pot of salted boiling water, add green beans and cook until bright green and crisp-tender, about 2-3 minutes.
Quickly drain and transfer to a prepared ice bath to cool, drain again and transfer the green beans to the casserole.
Heat oil in a large nonstick pan over medium-high heat. Add mushrooms and cook, stirring occasionally for about 5 minutes.
Add garlic and season with oregano, salt, and pepper, and continue to cook for a minute or so, or until garlic is fragrant and mushrooms are golden. Transfer to the casserole, mixing it with the green beans.
Meanwhile, in a saucepan whisk milk, cornstarch, a pinch of salt and pepper. Bring it to the boil, and stir in the cheese. Continue to simmer for a couple of minutes, until sauce thickens and cheese has melted.
Remove from heat and add it to the green bean casserole, and toss until combined. Sprinkle with panko and place it into the preheated oven.
Bake at 350 F until bubbly and golden on tops, about 10-12 minutes. Enjoy!
Notes
Using fresh green beans - instead of frozen - gives you the crispiest beans.
To make this green bean casserole Gluten-free, use gluten-free panko/breadcrumbs.
For a spicy kick add some chopped jalapenos or a sprinkle of crushed red pepper.
If the cheese mix is too thick, thin it out with a splash or two more of milk.
Use dairy-free cheese to make this dish dairy-free.
Green beans can be blanched 1 day in advance and the sauce prepared 1 day in advance.