Whisk together mustard, sage, salt, pepper, garlic and vinegar. Whisk in oil. Add pork and marinate for at least 30 minutes.
Put pork on grill on direct high heat. Cook on high for ~4 minutes before flipping. Flip and turn heat down to medium, line upright side with sage leaves. Continue cooking, turning occasionally until the interior reads ~155°F Remove from heat, cover, and let rest for 10 minutes. Slice and serve.