Sardines are the key to a modern reinterpretation of this breakfast classic. Sardines are economical, sustainable and rich in omega-3. Avocados are also full of healthy fats, making them a seemingly modest meal, but one that will keep you going for hours.
In a small bowl, combine the sardines, cilantro, mayonnaise and 2 1/2 teaspoons of lime juice. Mash together thoroughly with a fork and season to taste with pepper and a pinch of salt, if needed.
Cut the avocado in half lengthwise and remove the pit. Using a large spoon with a wide, shallow bowl, scoop each avocado half from its skin: run the tip of the spoon around the edge of the avocado to loosen it, then plunge the spoon between the skin and flesh. Keep the back of the spoon as close to the skin as possible, and work from the wider to the narrower end of the avocado. Rub each half with the remaining 1/2 teaspoon lime juice.
To serve, scoop half of the sardine salad into the hollow of each avocado. Serve over dressed salad greens if desired.