Best Tuna Noodle Casserole
1 (12 ounce) package egg noodles 1 (8 ounce) package sliced fresh mushrooms 1 cup frozen sweet peas 1 cup panko bread crumbs 1 small white onion, minced 1 teaspoon Dijon mustard 1 ½ cups chicken stock 1 ½ cups shredded white Cheddar cheese 2 (6 ounce) cans tuna, drained 2 cups milk 2 tablespoons finely chopped fresh parsley 2 tablespoons lemon juice 4 tablespoons salted butter, divided 4 ¼ teaspoons kosher salt, divided 5 teaspoons olive oil, divided ¼ cup all-purpose flour ¼ teaspoon ground paprika ½ teaspoon ground black pepper