Preheat oven to 400°F and line a baking sheet with parchment paper.
Peel and dice the sweet potato into 1/2-inch cubes.
Toss sweet potato cubes with half the olive oil, sea salt, and black pepper, then roast for 20-25 minutes until tender and golden.
While potatoes roast, drain the canned tuna and place it in a medium mixing bowl.
Finely dice the red onion and cucumber, and halve the cherry tomatoes.
In a small bowl, whisk together the remaining olive oil, lemon juice, and dried oregano to create the dressing.
Combine the tuna, roasted sweet potatoes, baby spinach, cherry tomatoes, cucumber, and kalamata olives in a large bowl.
Drizzle with the lemon-herb dressing and toss gently to combine before serving.