In recent years, the world of education has become increasingly Hybrid (online / on location) and Flexible (synchronous / asynchronous). One of the risks of these mixed environments is the distance between teacher and students that can make interaction, a crucial component of the teaching / learning process, more difficult. This paper introduces Evoli, a tool to support the "HyFlex model"; more specifically, the component dealing with online / asynchronous mode. Evoli enables teachers to receive precise, time-stamped feedback by their students on educational materials (typically videos). Students go through the materials and express their level of understanding as well as their questions and comments. Dashboards with the students' data allow the teacher to know, topic by topic, what is clear and what is not and thus how to organize the synchronous sessions. The tool was evaluated in a real-life setting, involving 63 graduate students in a course on Plasma Physics. The students filled in a System Usability Scale questionnaire and some questions regarding the perceived usefulness of the tool; the teacher's opinion was gathered via a semi-structured interview. Results show that students found the tool both usable and useful; the teacher's opinion was that the tool allowed prompting more reactions than a normal setting and an optimization of teaching organization.