Certified online course for learning STEM subjects?

I'm thinking of switching fields within STEM and there are some mathsy modules which I missed out on during my undergrad (biology) that would come in really handy right now.
Since I would like to avoid doing another bachelor's from scratch, I was hoping there might be a website that lets you pick and choose from a range of undergrad-level subjects that you would take online, and then possibly give you a certificate that you could put on your CV.
Does anyone know if something like this exists?