Bring a large pot of generously salted water to a boil. Cook the spaghetti according to package directions until just al dente, about 8 to 9 minutes. The pasta should have a tiny bit of bite left because it will finish in the skillet. Before draining, scoop out about a quarter cup of the starchy cooking water. Drain the pasta and set aside.

While the pasta cooks, open the tuna cans and drain them into the skillet, collecting all the oil from the cans. Add the 3 tablespoons of olive oil to the tuna oil already in the skillet. Set the skillet over medium heat.

Once the oil mixture shimmers, add the minced garlic and red pepper flakes if using. Cook for about 60 seconds, stirring, until the garlic is fragrant. Add the zucchini, salt, and black pepper. Cook for 4 to 5 minutes, stirring occasionally, until the zucchini is tender and has some golden color on the edges. It should be soft but not mushy.

Add the capers and stir them in. Then flake in the drained tuna in large chunks, breaking it apart gently with a fork or spoon. You want some texture in the pieces, not a paste. Stir and cook for just 1 minute so the tuna warms through without drying out.

Add the drained spaghetti to the skillet along with the lemon juice and lemon zest. Use tongs to toss everything together well. If the pasta looks dry or sticky, add the reserved pasta water a tablespoon at a time, tossing after each addition, until the pasta is glossy and coated in the pan juices. Taste for salt and lemon, adjusting as needed.

Remove from heat and scatter the fresh parsley over the top. Toss once more and serve immediately. Parmesan or pecorino cheese on the side is optional but very good if you want it.