In a bowl, whisk 1 teaspoon of lemon juice, 1 teaspoon olive oil and a pinch of salt.
Add in the avocado to the bowl with the lemon dressing. Use the back of a fork to break the avocado into smaller pieces, then mash into dressing until well combined and mostly smooth. Set aside.
In another bowl, add in the sardines. Break into smaller pieces with the back of a fork.
To the bowl with the sardines, add the tomato, chile, red onion, sardines oil, juice of ½ lemon, oregano and salt (to your preference). Mix in to combine. Set aside.
Coat one side of each of the slices of bread with the remaining teaspoon of olive oil.
Toast the bread in a toaster until golden on both sides (about 1-2 minutes).
If toasting on a pan, set the pan at medium to high heat. Once the pan is warm, toast bread for 2 minutes on each side or until golden brown on both sides.
Spread avocado mixture evenly on side of bread coated with olive oil. Lay a thin layer of arugula on top. Finish by spreading the sardines mixture evenly on the arugula.
Garnish with chopped cilantro, optional.