Boil the pasta: Bring a large pot of salted water to a boil. Add the pasta and cook until al dente according to package directions.
Reserve 1 cup of the starchy cooking water, then drain.
Prep the creamy base: In a small bowl, whisk together the Greek yogurt, lemon zest, lemon juice, Dijon, a pinch of salt, and a few grinds of pepper. Set aside.
Sauté aromatics: While the pasta cooks, heat the olive oil in a large skillet over medium heat. Add the onion and cook 3–4 minutes until softened.
Stir in the garlic and red pepper flakes, and cook 30 seconds until fragrant.
Add tomatoes and seasoning: Stir in cherry tomatoes and oregano. Cook 2–3 minutes, just until the tomatoes start to break down and release juices.
Fold in tuna and greens: Add the drained tuna, breaking it into big flakes. Stir gently to warm through, about 1 minute.
Add the spinach and cook until just wilted, 30–60 seconds.
Combine with pasta: Add the drained pasta to the skillet. Toss to coat. Sprinkle in the Parmesan and 1/4 cup reserved pasta water.
Toss again until glossy.
Make it creamy: Remove the skillet from the heat. Stir in the yogurt-lemon mixture. Add more pasta water, a splash at a time, until the sauce clings to the pasta and looks silky.
Taste and adjust salt, pepper, and lemon.
Finish and serve: Top with chopped parsley or basil and extra Parmesan if you like. Serve warm.