Slice the onion thinly and add it to a skillet with a little olive oil over very low heat. Add salt and a tiny pinch (just a little) of baking soda. This helps release the sugars and caramelize the onion faster. Cover and cook slowly until caramelized.

Place the sun-dried tomatoes in a blender or food processor. Add sea salt, black pepper, and a splash of extra virgin olive oil. Blend until it forms a paste.

Assemble the sandwich. I used a whole wheat baguette, but use any bread you like. Slice the bread open and spread the tomato paste on it.

Drizzle with a generous amount of extra virgin olive oil and arrange some sardines on top.

Add caramelized onions and sliced olives, close the sandwich, and cut in half.