In a wide, cast-iron skillet, combine the fennel, onion, sunflower seeds, olive oil and salt and cook over really low heat, stirring occasionally, until everything has a nice golden-brown color, 2 to 3 hours. (The goal here is to slowly caramelize the fennel and onion evenly, while also slow-toasting the sunflower seeds. Take your time and do not be tempted to speed up the process. Slow and low is good here.

Meanwhile, wash the raspberries thoroughly in a bowl of water.

Gently lift the raspberries from the water with your hands and let them dry on towels. Once dry, place them in a bowl and roughly crush them manually with hands.

Add the raspberries and any resulting juices to the pot of caramelized fennel, onion and sunflower seeds. Continue to cook over low heat until the sofrito is dark, jammy and savory, another 3 hours.

Season with additional salt if needed, then season to taste with pepper.

Let cool to room temperature. Store airtight in the refrigerator for up to 1 week.