Combine first 3 ingredients in a food processor; process until smooth. Spoon into a bowl; stir in dill and onion.
Divide salmon mixture evenly between 2 bread slices; top each with 2 tomato slices, 4 cucumber slices, and 1 bread slice. Store in small zip-top bags in refrigerator.
Made this sandwich on toasted everything bagels with roasted potatoes and a green salad for an easy dinner. Really delicious! Pumpernickle or sesame bagels would be good, too.