Green Bean Casserole With Mushroom Bechamel & Crispy Onions

Ingredients

• cooking oil, such as peanut or canola oil
• 1 cup all-purpose flour
• 1/2 cup lager-style beer, plus more as needed
• 1 small sweet onion, sliced into very thin rings or half moons
• kosher salt
• 2 pounds green beans, ends removed and beans snapped in half
• 6 tablespoons unsalted butter, divided
• 1 pound baby portobello mushrooms, cleaned and diced
• 1 small sweet onion, diced
• 4 cloves garlic, peeled and minced
• 1/2 cup dry white wine
• 1 cup whole milk
• 1 cup heavy cream
• 1 cup chicken stock
• 1/4 cup plus 2 tablespoons all-purpose flour
• 1 teaspoon worcestershire sauce
• 1 teaspoon ground mustard powder
• kosher salt and pepper, to taste
• 1 cup freshly grated sharp cheddar