Green Bean Casserole from Scratch - Mother's Bistro

Ingredients

• 2 large yellow onions, peeled and cut into ¼-inch-thick slices ((about 4 cups))
• 4 ¼ cups whole milk, or more to cover the onions if necessary
• 1 cup all-purpose flour
• 1 teaspoon garlic salt
• ¼ teaspoon salt
• ¼ teaspoon black pepper
• 6 cups (1 1/2 quarts) vegetable oil, for frying (Approximately)
• 1 ½ teaspoons salt (divided), plus more for salting blanching water
• 2 pounds fresh green beans,stem end trimmed, left whole or cut in half crosswise ((about 8 cups))
• 6 Tablespoons unsalted butter ((divided))
• ¼ cup all-purpose flour
• 1 pound sliced button mushrooms ((about 3 cups))
• ½ teaspoon freshly ground black pepper ((divided))