Bring a large pot of water, seasoned generously with salt, to a rolling boil.

Meanwhile, place a large skillet or Dutch oven over medium heat (Note: this vessel must be large enough to accommodate all the ingredients including the cooked pasta).

Add the olive oil and once shimmering, add the fennel, stirring occasionally until it begins to soften, being careful not to brown it.  Meanwhile, add the spaghetti to the boiling water and cook until al dente.

Add the onion to the fennel, a few pinches of salt, and cook until onions are translucent. Reduce heat to medium/low and add white wine, stirring constantly as the liquid reduces. Repeat one by one with vinegar and lemon.

Stir in capers and red pepper flakes. Add sardines and mash up with the back of a wooden spoon, stirring until incorporated. Turn heat to medium/high and add three ladles of pasta water to the pan, stirring continuously, reducing liquid until a sauce begins to form.

Add pasta to the pan along with one more ladle of pasta water, mixing with tongs to combine ingredient until glossy sauce coats the spaghetti. Removed from heat, stir in most of the fennel frond (saving a few choice sprigs for plating) and combine.

Plate in your favorite bowls, topping with remaining fennel frond and lemon zest. Drizzle on more EVOO and sprinkle some sea salt to finish. Enjoy!Serves 2-4Keeps in fridge for up to one weekTo reheat: Add cold pasta to hot pan with ¼ cup water per serving. Stir gently until glossy sauce reforms.