Bring a big pot of water to a boil, add the rock salt then follow with the dry spaghetti. Cook the spaghetti following the number of minutes suggested in the package (usually it is about 10 minutes). Lessen 1 minute from the number of minutes suggested.
In a large saucepan in medium fire, warm up ⅓ of the extra virgin olive oil. When it's hot, add the garlic, leaving it to turn lightly golden and aromatic. Discard.
Add the tomatoes and toss for about 6 minutes.
Add the garum and lemon juice then turn off the fire.
Mix in the cooked spaghetti to the sauce then add the remaining ingredients: lemon zest, parsley and remaining extra virgin olive oil.
Serve immediately.
