In a medium bowl, combine the diced avocados, scallions, peppers, and garlic. In another bowl, whisk together the orange and lime juices, olive oil, and salt and pepper. Pour over the avocado mixture and gently toss. Refrigerate for 15 minutes.
Meanwhile, for the cucumbers: Mix the sugar and vinegar in a bowl, stirring well. Add the cucumbers and allow to marinate for about 15 minutes.
To serve, arrange the cucumber in neat slices across each plate. Slice the tuna and lay it over the cucumbers. Spoon the avocado salsa on top of or around the tuna. Spoon a little of the pickling juices around the cucumbers, and serve. (Sometimes I add a little grated orange zest for garnish as well.)
For the adobo:
Mix all of the ingredients together in a bowl.
Rub each of the tuna steaks with 1 1/2 teaspoons of the oil and sprinkle generously with the adobo. (Reserve any extra adobo spice rub for another use.) In a nonstick skillet, heat the remaining 2 tablespoons oil until it begins to smoke. Sear the tuna on each side for only 1 minute — the tuna is served rare. Transfer to a plate and set aside.