Dice the onion and rinse in a strainer under warm water. Cut the tomatoes in half, crosswise and squeeze out the seeds. Dice the tomatoes and place them in a large bowl with the salt. Cut the stems off the serrano chiles (add less chiles if your like a milder sauce or more if you like it hotter), then slice into very, very thin rings and add to the tomatoes. Add the onions to the bowl of tomatoes, and chiles. Roughly chop the cilantro springs and add to the bowl. Add the lime juice or apple cider vinegar, and the cracked black pepper (optional), and stir well. Let it set for at least 30 minutes to let all the flavors mingle. After 30 minutes, retaste the pico de gallo, add more salt or lime juice or apple cider vinegar according to your taste.