Elementary teachers are underprepared to teach mathematics, and there is a lack of field-based support for mathematics-specific pedagogies in the elementary grades. To address this theory to practice gap, we developed an innovative model of fieldwork that draws on the expertise of in-service teachers (elementary mathematics specialists [EMSs]) who had recently completed a K-5 mathematics endorsement to work in the role of university supervisors supporting beginning teachers (BTs) in initial fieldwork. We argue that this model has three key aspects that will support BTs bridging the theory to practice gap: (1) as in-service teachers the EMSs are keenly connected to the context of schools; (2) recent experience in university coursework in mathematics while serving as in-service teachers required the EMSs to navigate the theory to practice gap themselves; (3) one-on-one mentorship supports strong and trusting relationships. Drawing on data from a 3-year study we found that EMSs brought intimate knowledge of the school context and knowledge of the mathematics-specific pedagogies taught at the university. These connections to the field and the university allowed EMSs to support BTs in implementing research-based practices in their mathematics lessons that went against the norms of their school settings.