1 medium onion, finely chopped
4 1/2 tablespoons unsalted butter
10 oz mushrooms, trimmed and sliced 1/4 inch thick (4 cups)
2 teaspoons soy sauce
1/4 cup Sherry
1/4 cup all-purpose flour
2 cups low-sodium chicken broth
1 cup whole milk
2 teaspoons fresh lemon juice
1/4 teaspoon salt
1 (6-oz) can tuna in olive oil, drained
6 oz dried curly egg noodles (preferably Pennsylvania Dutch style; about 3 1/4 cups)
1 1/2 cups coarse fresh bread crumbs (from 3 slices firm white sandwich bread)
4 oz coarsely grated Cheddar (1 cup)
1 tablespoon vegetable oil