For the casserole:

1 tablespoon olive oil
1 tablespoon unsalted butter
2 celery stalks, small dice (about 2/3 cup)
1 bunch scallions, white and light green parts only, thinly sliced
1 tablespoon minced jalapeño (no seeds or membranes)
1 teaspoon Old Bay Seasoning
2 tablespoons flour
1 3/4 cups milk
1/4 cup sour cream
1 tablespoon Dijon mustard
6 ounces flat egg noodles
3/4 cup coarsely grated aged cheddar cheese (about 3 ounces)
6 ounces high-quality imported tuna packed in oil, drained

For the topping:

1 tablespoon minced parsley leaves
2 tablespoons unsalted butter, at room temperature
1/4 cup coarsely grated aged cheddar cheese (about 1 ounce)
1 cup crushed saltines (about 20 whole crackers)