Chop the cucumber and red onion into small cubes.
In a bowl, mash the vegan tuna with a fork until it separates into small pieces. Mix in cucumber and red onion chunks.
Add vegan mayo and parsley, mix until fully combined.
Toast two slices of your favorite bread.
Spread vegan mayo on each piece of toast, to your preference.
Spread tuna salad mixture on one slice of bread, top with lettuce, then press together with the other slice of bread. Cut diagonally into two triangles.