The growing adoption of English-medium instruction (EMI) in multilingual higher education contexts raises concerns about faculty readiness, particularly regarding language proficiency, pedagogy, and institutional support. Even experienced lecturers often feel underprepared for EMI, underscoring the need for structured professional development. At the Universidad de Las Palmas de Gran Canaria, a two-year institutional project (2022–2024) was implemented to strengthen EMI practice through structured mentoring and peer teaching. The initiative engaged 25 educators from EMI and from English for Specific Purposes (ESP) and English for Academic Purposes (EAP) disciplines in reciprocal classroom observations. ESP/EAP mentors provided rubric-based feedback on EMI practices—including instructional design, communicative competence, and intercultural awareness—while EMI lecturers observed ESP classes, first offering open-ended reflections and later rubric-based feedback and peer discussions. A mixed-methods approach combined quantitative rubric analysis with qualitative feedback. Findings indicate EMI strengths in specialized language use, discourse strategies, and digital material design, with classroom interaction and intercultural competence identified as areas for growth. While mentors emphasized classroom management, EMI lecturers prioritized instructional design and student engagement. This collaborative framework offers strategies for targeted professional development, providing a transferable model to support faculty and advance EMI practices in increasingly internationalized universities.