Combine tuna, olive oil and fresh herbs in a small bowl. Season to taste with salt and pepper.
Arrange sliced avocado over toast, top with tuna mixture and pickled onions.
Drizzle with lemon juice and garnish with extra fresh herbs. Serve immediately.
If using tuna in oil, omit olive oil.
To make quick-pickled onion: thinly slice half a red onion and toss in a bowl with ½ tsp salt ad 2 tsp apple cider vinegar. Allow to sit for 30 minutes, stirring regularly, until soft. Onions can be made in advance and stored in a sealed jar in the fridge for up to a week.
To pick a ripe avocado, press gently near the top. If it gives a little, it’s ripe.