Pat the white fish and tuna dry with paper towels. Cut the white fish into 1/2" cubes and place in a medium nonreactive bowl. Keep the tuna cubes separate in a small bowl and refrigerate; tuna will be served mostly raw and should not be fully cured in citrus.

Make citrus marinade

In a bowl, combine the lime juice, lemon juice, orange juice, sea salt and sugar. Stir until salt and sugar dissolve. Taste and adjust acidity/salt balance — it should be bright and slightly salty.

Marinate white fish

Pour 3/4 of the citrus marinade over the white fish so the pieces are mostly submerged. Cover and refrigerate for 12–20 minutes until the fish becomes opaque and slightly firm to the touch (it should be 'cooked' through by the citrus but still tender). Do not over-marinate — small pieces will finish in this time range.

Drain and combine

After the white fish has cured, drain off most of the marinade, reserving about 2 tablespoons. Transfer the cured white fish to a large mixing bowl. Add the reserved marinade to the reserved fresh tuna cubes (this will lightly dress the tuna) and toss gently to coat; let tuna rest in the dressing for 2–3 minutes.

Add vegetables and aromatics

To the bowl with cured white fish, add the thinly sliced red onion (if too pungent, rinse briefly in cold water and drain), minced jalapeño, diced cucumber and chopped cilantro. Toss gently to combine. Drizzle 1 tablespoon extra-virgin olive oil, sprinkle freshly ground black pepper, and adjust salt to taste.

Finish with tuna and avocado

Fold the lightly dressed tuna into the fish-and-vegetable mixture very gently so tuna retains its texture. Gently fold in diced avocado just prior to serving to avoid discoloration.

Finishes & Garnish

Taste final ceviche and adjust seasoning — add a touch more lime juice, salt, or a dash of the optional toasted sesame oil or soy/ponzu if you want an umami note for the tuna.

Spoon the ceviche into a chilled shallow bowl or onto a platter. Garnish with microgreens or cilantro sprigs and a light drizzle of olive oil.

Serve immediately with tortilla or plantain chips. Ceviche is best eaten the day it is prepared; any leftover should be kept refrigerated and consumed within 24 hours (note that texture will change as citrus continues to cure the fish).