Drain the sardines thoroughly and place them in a mixing bowl. Use a fork to gently break them into small chunks. Avoid mashing completely—you want some texture.

Cut the avocados in half, remove the pit, and scoop the flesh into the bowl with the sardines. Lightly mash with a fork, leaving some chunks for texture.

Add lemon juice, olive oil, Dijon mustard (if using), garlic, black pepper, and red pepper flakes. Mix gently until everything is combined but not overly smooth.

Taste the mixture and add salt only if needed. Sardines can vary in saltiness, so this step is important.

Fold in freshly chopped herbs. This adds freshness and balances the richness of the fats.

Slice cucumbers into thick rounds or separate endive or lettuce leaves. Arrange them on a serving platter.

Spoon the sardine-avocado mixture onto each cucumber slice or leaf. Garnish with capers, olives, or extra herbs if desired.