1 1/2 cups panko
9 tablespoons unsalted butter
1 medium yellow onion, small dice (about 1 1/2 cups)
2 pounds green beans, ends trimmed and cut into 1-1/2-inch pieces
Freshly ground black pepper
12 ounces cremini mushrooms, stems trimmed and cut into small dice
1/3 cup all-purpose flour
4 cups (1 quart) whole milk, at room temperature
1 1/2 teaspoons kosher salt, plus more for seasoning