1. In a heavy saucepan over medium-high heat, melt the butter. Add the onion and garlic and sauté until translucent, about 10 minutes. Add the remaining sauce ingredients and simmer for about 15 minutes. Continue cooking until the sauce begins to thicken, about an additional 20 to 30 minutes. Taste and adjust seasonings with salt and pepper.
2. Let the sauce cool for about 10 minutes, or until it is warm but no longer hot. Using an immersion of traditional blender, puree the sauce. Let cool and, if not using immediately, pour into a clean glass jar. The sauce can be made in advance and refrigerated for 2 weeks.
3. To grill the pork, heat the grill to medium.
4. Wrap the tenderloins in paper towels to remove any surface moisture. This will help to get great grill marks. Brush the dry meat with olive oil, then season with salt and pepper. Place the meat directly over the heat to sear. Grill for 2 to 3 minutes, turning once or twice, or until good grill marks appear.
5. Switch to medium indirect heat and move the meat to the center of the cooking grate and cook for 30 to 35 minutes total, turning once during cooking and brushing with the barbecue sauce every 5 to 7 minutes until the tenderloins are done and the sauce has created a glaze. Using a meat thermometer, make sure the tenderloins read 145 degrees at the thickest part.
6. Remove the tenderloins from the grill, allow them to rest for 5 to 10 minutes, then slice on the bias.
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