Bring about 2 inches of water to a boil in a large skillet and salt it.
Trim the green beans and cut into 2-inch pieces. Halve the lime; squeeze the juice into a large bowl.
Add 1/4 cup olive oil and 1 tablespoon soy sauce to the bowl and whisk to combine.
Trim, peel, halve, and thinly slice the onion; add it to the bowl. Halve and pit the avocados, scoop out the flesh, cut into chunks; add them to the bowl. Prepare a bowl of ice water big enough to hold the green beans.
When the water comes to a boil, add the green beans, cover, and cook until they’re just tender, about 3 minutes.
Drain the beans and immediately transfer them to the ice bath.
Break the tuna into chunks and add it to the bowl. Toss gently, adding more olive oil if the salad looks too dry. Taste and adjust the seasoning and serve.