Fresh Green Bean and Potato Casserole (Creamy, Crispy Topped) - Quickie Recipe

Ingredients

• 1 1/2 lb (680 g) Yukon Gold potatoes, scrubbed and cut into 3/4-inch pieces
• 1 1/4 lb (565 g) fresh green beans, trimmed and cut into 2-inch pieces
• 1 tablespoon kosher salt, for the boiling water
• 3 tablespoons (42 g) unsalted butter, plus more for the baking dish
• 1 small yellow onion, finely chopped
• 8 oz (225 g) cremini mushrooms, sliced
• 3 cloves garlic, minced
• 3 tablespoons all-purpose flour
• 1 1/2 cups (360 ml) whole milk
• 3/4 cup (180 ml) vegetable broth
• 1 teaspoon chopped fresh thyme, or 1/2 teaspoon dried thyme
• 3/4 teaspoon kosher salt, plus more to taste
• 1/2 teaspoon black pepper
• 1 cup (115 g) shredded sharp cheddar cheese, divided
• 1 1/4 cups (70 g) crispy fried onions