1. To make the filling, place the colander over the mixing bowl. Empty the tuna into the colander so the liquid drains into the bowl. Throw away the liquid.
2. Put the drained tuna into the bowl. Break it into chunks with the fork. Stir in the mayonnaise or salad dressing and relish.
3. For each sandwich, spoon about one-third of the filling on one slice of bread. With a table knife, spread the filling to the edges of the bread. Tear lettuce leaves to fit the bread. Place some lettuce on top of the filling. Put another slice of bread on top. Cut in half. Repeat for each sandwich.