This case study describes a Q methodology study which captured and analyzed the viewpoints of 45 UK teachers online. The teachers might liken their participation in the study to a "card sort" activity: their relative placement of statement cards revealed their opinions of mastery in mathematics. Factor analysis of the completed sorts yielded the different viewpoints held in common. Freely available software was used to conduct the entire research study online. This online approach allowed a diverse set of teachers to be anonymously recruited, and these teachers completed their sort in a time and place convenient for them. The results of the described study identified four distinct viewpoints about a relatively new, and thus under-researched, approach to teaching mathematics which is known in England as "mastery." The online approach successfully addressed logistical challenges and resource considerations inherent in face-to-face data collection. The case study highlights the unique positioning of Q methodology within educational research. Readers of this case study are asked to consider online Q methodology studies as a rigorous, useful, and powerful addition to more conventional forms of mixed method research. This is especially pertinent in the current climate: the COVID-19 pandemic will continue to reduce physical interaction and has also increased teachers' familiarity and expertise with online activities. [This content is provided in the format of an e-book.]