Deficit mindsets of multilingual writers must be upended. Yet, little research has been done to explore how to prepare bilingual teachers with strength-based orientations, especially in approaches to assessment. Using a holistic biliteracy lens, this qualitative case study explored how Transliteracy observation shaped teachers' understanding of biliterate writing development in K-5 dual language settings. Findings showed that teachers learned that writing is multifaceted and observable, influenced by oral language, and reflective of one linguistic repertoire. Participants gained a deeper understanding of biliteracy development and their students' capabilities, supporting an asset-based orientation that challenged monolingual norms. Implications for teacher preparation are shared.