Pre-heat oven to 400 degrees F
Season tenderloins with salt and pepper and coat with ¼ cup of whole grain mustard.
Heat oil over med-high heat in a large oven-safe pan (I like to use cast iron). Once oil is hot, add pork and brown on all sides, turning to brown on all sides, ~6 minutes total.
Place in the oven and bake uncovered at 400˚F for 10-12 minutes, turning the tenderloin over halfway through baking. Bake until center of pork registers 130 to 135 degrees F then transfer to a cutting board and let meat rest 5-10 minutes.
To the pan, add remaining ¼ cup whole grain mustard, Dijon, and sour cream. Whisk over medium-low heat until warmed through. Add water to adjust consistency of sauce if desired.
Slice pork and serve with mustard pan sauce.