Preheat the oven to 400°F. Line a baking sheet with parchment paper and set aside.
Press the Sauté button on the Instant Pot. Melt 1 tablespoon of butter. When the butter starts sizzling, add the onion and cook for 3-4 minutes, until translucent.
3 tablespoons unsalted butter, ½ cup onion
Add the mushrooms and cook for 5-6 minutes, until they have softened and released most of their liquid. Add the garlic and cook for 30 seconds, until fragrant. Season with salt and pepper.
8 ounces sliced mushrooms, 2 cloves garlic, Kosher salt, Black pepper
Add the broth and green beans and stir until coated. Press the Cancel button.
1 cup vegetable broth, 1½ pounds green beans
Secure and seal the lid. Cook on Manual High for 4 minutes, performing a quick release of the excess steam when the timer goes off.
Meanwhile, spread the fried onions on the prepared baking sheet. Bake for 5-7 minutes, until they’re crisp and golden brown. Set aside until ready to use.
1 cup French fried onions
Melt the remaining 2 tablespoons of butter. In a small bowl, mix together the flour, parmesan cheese, and melted butter.
2 tablespoons all-purpose flour, 2 tablespoons freshly grated Parmesan cheese
When the pin drops on the Instant Pot, carefully remove the lid. Press the Sauté button. Stir the flour mixture into the beans, followed by the mushroom soup. Stir gently until well combined, adding a splash of broth if it’s too thick.
10.75 ounces cream of mushroom soup
If desired, transfer the green bean casserole filling to a baking dish, sprinkle the crispy onions on top, and serve.