Preheat your oven to 400°F and line a large baking sheet with parchment paper.
Slice the zucchini into half-moons, chop the red bell pepper into bite-sized pieces, and thinly wedge the red onion.
Place the zucchini, peppers, and onions on the baking sheet, drizzle with 0.5 tbsp of olive oil, and sprinkle with sea salt and black pepper.
Roast the vegetables for 20 minutes until they are tender and slightly charred around the edges.
While the vegetables roast, drain the canned tuna and flake it into a large mixing bowl using a fork.
In a small jar or bowl, whisk together the remaining 0.5 tbsp of olive oil, lemon juice, minced garlic, and dried oregano.
Add the roasted vegetables, halved cherry tomatoes, and sliced Kalamata olives to the bowl with the tuna.
Pour the lemon-oregano dressing over the mixture and toss gently until everything is well-coated and fragrant.