1. In a blender, combine celery, cucumber, Serrano chile, cilantro, water, salt, and pepper, and blend until smooth.
2. On a platter, arrange halibut and spoon blended mixture over. Garnish with red onion, avocado, and árbol chile. Let sit for 10 minutes.
3. Evenly mound fish aguachile on each Northgate® homemade tostada and pair with a refreshing drink.