Prepare the vegetables: thinly slice the onion, carrot, and cucumber. Soak the onion in cold water for 10 minutes to remove its sharpness.

Drain the canned tuna and press gently with a spoon or ladle to remove excess oil.

Combine the prepared vegetables and tuna in a mixing bowl.

Add 5 tbsp mayonnaise, 2 tbsp honey mustard, and 1–1.5 tbsp lemon juice to the bowl.