Make the marinade: combine 1 clove minced garlic, olive oil, cider vinegar, half teaspoon of salt and rosemary in a bowl.
Rinse and pat dry the pork tenderloin. Place the pork in a large, sealable plastic bag. Pour in the marinade, and toss gently to combine. Put in the refrigerator to marinate for 30 to 60 minutes, taking out for the final 15 minutes of marinating prior to cooking.
Preheat oven to 400 degrees on the convection setting. If not available, heat to 425 conventional. When the oven is ready, place the pork tenderloin in a roasting pan, and place in the oven.
The pork tenderloin will likely take about 20 minutes to roast, but check it after 15 minutes to avoid over cooking. When ready, pork tenderloin should still have a little pink showing, registering between 140 and 145 degrees when you remove it from the oven.
When the pork has done roasting, remove it from the oven. Place the pork on a platter to rest for a few minutes.
As the pork tenderloin roasts, make the blackberry Merlot sauce. Melt the butter in a small sauce pan on medium heat. Add the garlic and sauté for about 1 minute, until the garlic becomes fragrant.
Add the Merlot and blackberry jam to the saucepan, whisking to combine. Bring the sauce to a boil, then reduce to a simmer. Add the rosemary, salt and pepper. Simmer for about 10 minutes to reduce the sauce. Keep warm as the pork finishes cooking. Stir in a tablespoon or two of the juice from the resting pork.
Slice the pork to serve. Plate 3 to 4 pork slices, topped with the blackberry Merlot sauce. Enjoy with a glass of Merlot!
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