Cut halibut into 1/2-inch cubes. Place in non-reactive bowl (stainless steel or glass) with 1 tsp. minced garlic, 8-10 slices Serrano pepper, 2 Tbl. fresh chopped cilantro and enough fresh lime juice to cover fish completely (about 4 limes). Cover with plastic wrap and marinate in refrigerator for 4 hours. Drain and discard marinade.
Meanwhile, bring a pan of water to boil and blanch shrimp and scallops for one minute. Drain and rinse under cool water. Cut each shrimp into 2-3 pieces and dice scallops.
Combine cold cooked halibut with shrimp, scallops, diced veggies and 1/2 cup fresh chopped cilantro. In a small bowl whisk together 1/4 cup olive oil with 4 Tbl. fresh lime juice (about 2 limes). Pour onto ceviche and toss to combine. Chill well, about 30 minutes.
To serve, cut avocados in half, discard pits. Scoop out some of the flesh, dice and add to ceviche. Fill avocado bowls with ceviche and serve with corn tortilla chips.