Mash avocado with garlic, crème fraîche, half of serranos, and 1 Tbsp. lime juice in a small bowl until mixture is mostly smooth; season with salt. Hold your knife at a 45° angle and slice fish with the grain into 1/2" slices (use a sharp blade and aim for one long, clean stroke). Cut each slice crosswise into 1/2" pieces. Combine oil, fish sauce, shallot, ginger, yuzu kosho, remaining serrano, and remaining 1 Tbsp. lime juice in a medium bowl. Add yellowtail, season with salt, and gently toss crudo to combine. Spread avocado mixture on plates. Top with yellowtail, radish, and cilantro. Serve with crackers.