Introduction: Clinical reasoning is a crucial competence for medical practice and also a complex task that is susceptible to cognitive errors. It is usually taught based on whole clinical cases, from a practical rather than from a conceptual perspective. Addressing the need to improve clinical reasoning teaching into undergraduate medical curricula, it was hypothesized that medical students could benefit from a practice and theoretical approach to the clinical reasoning process. Methods: A four week online course, based on simulation and groupal reflective practice was developed, to promote metacognition between the participants. The course was delivered to 8 sixth year medical students as an elective module. A questionnaire consistent of four open-ended questions was designed to explore knowledge about clinical reasoning and cognitive errors, and was applied at the beginning and at the end of the course. A qualitative analysis was conducted with Berelson content analysis method. Results: After the course the students changed their understanding about clinical reasoning, considering it more like a process and identifying the dual nature described in modern theories of clinical decision making. Also they changed their knowledge about cognitive errors, attributing them not only to lack of knowledge, and understanding that they can actively practice some strategies to reduce cognitive bias. Discussion: This study confirmed that undergraduate students change in a positive way their concept of clinical reasoning and their knowledge about this cognitive process and cognitive errors that occur on it after a course that includes simulation and reflection.