Our experience with grass-fed meat shows that it likes to be cooked on low heat. We use to grill everything but now do it on the stovetop or in the oven since we can control the heat better.
Grass fed beef needs to be completely thawed and at room temperature before you start to cook it. It does not transition well from just thawed to a searing pan.
We like to sear them on high heat for no more than 2 seconds on each side then put them in a pan and finish cooking them in the oven at 300°. We prefer to use a cast iron skillet. They will cook in approximately 15 to 20 minutes. This will give you a meat that is rare. You may need to cook a little longer depending on your preference. We love our grass-fed meat rare!
Again, we like to start off by searing them on high for no more than 2 seconds each side, then turn them down low. Continue cooking until desired preference. Try to take the burgers off a little earlier than you normally would, as they will continue to cook.