Mix the dressing. In a small bowl or jar, whisk together olive oil, lemon juice, Dijon, garlic, oregano, salt, and pepper until slightly thick and glossy. Taste and adjust with more lemon, salt, or pepper as needed.
Prep the tuna. Drain the tuna well.
Flake it gently with a fork so you get bite-sized pieces. If using tuna in water, add a teaspoon of olive oil for richness.
Build the base. Add romaine or mixed greens to a large bowl. Scatter on tomatoes, cucumber, red onion, chickpeas, olives, and roasted red pepper if using.
Add the tuna. Place the flaked tuna over the vegetables.
Sprinkle with feta and chopped herbs.
Dress and toss. Drizzle about two-thirds of the dressing over the salad. Toss gently with tongs until everything is coated but not soggy. Add more dressing if you like.
Finish and serve. Taste and adjust seasoning.
Add lemon wedges on the side for extra brightness. Serve with warm pita or crunchy croutons if you want a heartier plate.
Optional upgrades. Top with quartered hard-boiled eggs or sliced avocado for extra protein and creaminess.