The promotion of high-quality written communication in the disciplines is an important learning outcome in higher education. Given the time invested by students and teachers alike, it is crucial that writing assignments also promote engagement and content learning. But is it worth the time for university teachers to invest in such 'writing-to-learn' activityes? We find that it can be, and present an improved design for an experimental lab-report writing assignment in an English medium instruction environment, where English is an additional language. Our context is assignment development for formative assessment in master's-level physics, but the method is broadly applicable within the science-technology-engineering-math disciplines. Our first experience with the assignment resulted in substandard lab reports, suggesting insufficient subject understanding and prompting this assignment design. We therefore focused on communicating the alignment of aims, learning objectives, instruction, assessment criteria, and feedback design, and developed simplified rubrics facilitating assessment fairness and efficiency. The revised assignment enhanced the learning of the subject matter and the writing quality over the four years of the study, indicated by clearly improved reports and relevant peer feedback comments. The learning activity also had an observable but less distinct effect on the students' exam performance.