In a blender, process the celery, onion, garlic, pickled jalapeño, lime juice and salt until smooth. Pass through a sieve and refrigerate.
Cut the fish in large cubes and season with salt and pepper. In a hot cast iron pan, sear the fish on all sides until golden brown.
In a small saucepot, heat gently the lime juice mixture gently for about 5 minutes and mix with the seared fish.
Garnish with chopped cilantro, sliced red onion and diced mango.
Serve the ceviche immediately with Saladitas crackers.