In a bowl, combine the cucumber, radishes and celery. Season with salt and pepper.
Spread the mayonnaise on the bread slices. Top with the veggie pâté. Garnish with the vegetables. Close the sandwiches.
If you need to prepare these sandwiches in advance, spread softened butter on the bread before the mayonnaise: It will create a protective layer to prevent the bread from getting soggy. Top with baby spinach, if desired.