Chef John's Tuna Noodle Casserole

3 tablespoons unsalted butter

½ cup finely diced yellow onion

1 ½ teaspoons kosher salt, divided

3 tablespoons all-purpose flour

4 cups cold milk

1 cup shredded white Cheddar cheese

1 (12 ounce) package dry egg noodles

2 (5.5 ounce) cans tuna packed in olive oil, drained and crumbled, or to taste

¾ cup frozen peas, thawed and drained

1 pinch cayenne pepper

¼ teaspoon freshly ground black pepper

¼ teaspoon Worcestershire sauce

½ cup plain bread crumbs

½ cup grated Parmigiano-Reggiano cheese