Bring a large pot of salted water to a boil, then cook the spaghetti according to the package instructions until it is al dente. Reserve 1/2 cup of the pasta cooking water before draining, then set the drained pasta aside.

While the pasta cooks, heat the olive oil in a large skillet or pan over medium heat. Add the sliced garlic and crushed red pepper flakes, cooking for about 1 to 2 minutes until the garlic is fragrant and lightly golden but not browned.

Stir in the crushed tomatoes, halved Kalamata olives, and drained capers, bringing the mixture to a gentle simmer. Cook for about 8 minutes, stirring occasionally, until the sauce begins to thicken slightly and the flavors meld together.

Gently break up the canned pilchards into large chunks and add them to the skillet, along with any tomato sauce from the can. Let them simmer in the sauce for 3 to 4 minutes until they are heated through, being careful not to break them up too much so they retain some texture.

Add the drained spaghetti directly to the skillet with the sauce, tossing everything together over low heat to coat the pasta evenly. If the sauce seems too thick, splash in some of the reserved pasta water a little at a time until a glossy consistency is achieved.

Season with black pepper to taste and garnish generously with chopped fresh flat-leaf parsley before serving hot straight from the pan.