A comprehensive STEM education for students is meant to develop critical thinking, problem-solving, creativity, collaboration, and self-directed learning. These are the main objectives of the BLOSSOMS Initiative (Blended Open-Source Science Or Math Studies), an MIT-sponsored program founded in 2008. MIT BLOSSOMS has an OER online repository of interactive, inquiry-based STEM video lessons at the high school level, created by university and high school educators in ten countries, including the United States. The program puts forward a novel approach to STEM teaching and learning in order to achieve the above-stated objectives, as well as to inspire students and enhance retention in the STEM pipeline. However, systemic barriers and the lack of empirical evidence have impeded its widespread adoption in high school education. In this paper, we will present MIT BLOSSOMS and examine major implementation challenges with an eye towards designing a Project-Based Learning (PBL) BLOSSOMS assessment procedure.