How to treat a strained mcl/lcl?

Rest, maybe meds. Rest..Or take it easy and allow it to heal. Gentle range of motion, stretching, heat to stimulate/increase blood flow, and some antinflammatory medications can help. If it is severe and you experience peristent pain etc...Check with your doc.
RICE. The acronym "rice" is generally the most appropriate initial treatment for these kinds of issues. If the below do not help to alleviate symptoms in the next week, please see your physician. R-rest i- ice c- compression, such as ace wrap. E- elevation to decrease swelling. Supplemented with over the counter pain relievers.