Preheat oven to 400°F and line a baking sheet with parchment paper.
Peel and dice the sweet potato into 1/2-inch cubes, then toss with 1 teaspoon of oil from the tuna can, sea salt, and black pepper.
Spread the potatoes on the baking sheet and roast for 20-25 minutes until tender and slightly caramelized.
While potatoes roast, bring a small pot of water to a boil, carefully add the eggs, and cook for 7 minutes before transferring to an ice bath.
Peel the eggs and slice them in half, then flake the tuna in a small bowl, reserving the remaining sunflower oil.
In a large bowl, whisk the remaining sunflower oil with lemon juice and dried oregano to create a light dressing.
Add the rucola to the bowl and toss to coat, then top with roasted sweet potatoes, flaked tuna, and the halved eggs.