Drain tuna and reserve oil. Break tuna into small pieces and set aside. Place half of the reserved oil in a medium bowl. Add lemon juice, red pepper flakes, salt and pepper, whisking to combine. Using a 3-inch round cookie cutter, place cutter on center of plate. Layer a quarter of the avocado inside cutter. Top with almost a quarter of the tuna, a quarter of the mango, and a quarter of the cucumber. Lightly press on top of cucumber while removing cutter. Repeat with remaining ingredients. Garnish tops with remaining tuna and a bit of mint. Drizzle dressing over the top. Serve immediately.