Squeeze the limes to extract the juice, ensuring it is freshly squeezed for the best flavor.
Combine the lime juice, salt, and sugar in a glass or non-reactive bowl. Stir until the salt and sugar are fully dissolved.
Add the minced garlic and chopped jalapeño or serrano pepper (if using) to the lime juice mixture. Stir to combine.
Add the finely chopped red onion to the marinade and mix well. Let the mixture sit for about 5 minutes to allow the flavors to meld together.
Finally, add the chopped cilantro leaves to the marinade and gently stir to incorporate.
The marinade is now ready to be used for your ceviche recipe. Pour it over the raw fish or seafood and ensure all the pieces are fully submerged in the marinade. Let the ceviche marinate in the refrigerator for about 15 to 30 minutes or until the fish or seafood appears "cooked" and opaque.
Once the ceviche is ready, strain off the excess marinade before serving. The strained marinade can also be used as a dressing or sauce to accompany the ceviche.