Step 1 Soak the prunes in the Cognac in a small bowl.
Step 2 Meanwhile, season the meat with salt and pepper. In a large skillet over medium-high heat, heat the oil and add the garlic. Cook until the garlic is rich golden brown, then discard. Add the pork slices to the skillet, in batches if necessary; do not crowd the pan.
Step 3Brown the meat quickly, 1 to 2 minutes, then turn and brown the other side. Remove the meat to a platter.
Step 4Add the onion to the pan and cook until tender, 3 to 5 minutes. Add the rosemary and sage and cook and stir 1 minute more. Reduce the heat to medium, add the prunes and Cognac and cook a minute so the alcohol evaporates.
Step 5Add the broth and stir, scraping up the brown bits, and cook until the liquid is slightly reduced, 1 to 2 minutes. Return the pork to the pan, spooning the sauce over and cook until heated through, 1 to 2 minutes. Add salt and pepper, if needed.