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!