The fillings cannot be anything that will make the bread soggy.
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I just made this for a plane trip: fresh ricotta cheese mixed with fresh thyme, lemon zest, salt, pepper. Slather a generous amount on each bread slice, top on bread slice with sliced cucumbers (used English cukes). Loved it especially since the ricotta was homemade. It was even better at room temp and no soggy bread
I like to put a thin layer of butter on the bread before adding the filling. When cold, it acts like a fat barrier, keeping the bread from getting soggy. My favorite make ahead tea sandwiches: roast beef with horseradish, smoked salmon and cream cheese, Branston pickle and cheese, chicken walnut apple salad.
Ina Garten has a tea sandwich that is amazing on a dense, date-nut type bread: thin layer of cream cheese, a slice or 2 of smoke turkey (very thin slices) and a basil leaf. To do these a day ahead, I would substitute arugula for the basil