Drain the oil from the tuna and reserve in a small bowl.
In a separate medium bowl, combine 1 tablespoon of the reserved oil, the vinegar, 1 tablespoon of parsley, lemon zest and fresh ground pepper, and whisk to combine. Add the drained tuna to the oil/vinegar/parsley mixture, and stir gently to coat the tuna.
Brush the slices of bread with some of the reserved oil and toast.
In a separate bowl, mash the avocado with a little salt. Spread 1/4 of the mashed avocado on each slice of toasted bread, and top with 1/4 of the tuna mixture. Sprinkle each piece with the remaining parsley, crushed red pepper flakes, a little more salt and pepper and serve with a lemon wedge.