Grilled Pork Tenderloin with Rhubarb Barbecue Sauce

Preparation

1. For the sauce: Place a small pot over medium heat for a minute, then drizzle in 1 tablespoon olive oil. Add onion and stir; when it starts to sizzle, reduce the heat to medium-low. Cook for about 5 minutes. Add garlic and cook, stirring occasionally, for 2 minutes, until softened and fragrant. Add rhubarb and stir. In a cup, stir the water and tomato paste until smooth, and pour into the pan. Add brown sugar, vinegar, Worcestershire, Dijon, chipotle powder, paprika and 1/2 teaspoon of salt. Stir until mixture starts to boil, and reduce heat to a low simmer. Cook, stirring frequently, until the rhubarb is falling apart and the mixture is thickened, about 5 minutes. Add more water if it seems too thick to be pourable. Let cool to room temperature while you grill the pork.
2. Heat the grill to high. Pat the pork dry and rub with remaining olive oil, salt and pepper. Use tongs and paper towels to rub oil on one side of the grill. Place the pork on the oiled grate and let cook, undisturbed, for 2 minutes, then carefully turn three times to cook each of the remaining sides for 2 minutes each. Turn the heat to low underneath the pork and cook for 4 additional minutes per side. Use an instant read thermometer to check the internal temperature at the thickest part of the meat. When the thermometer reads 160°F, take it off the heat and place on a clean platter. Let pork stand, loosely covered, for 10 minutes before serving.
3. Slice the pork crosswise in 8 slices, place on a warmed serving platter, then pour the rhubarb sauce over the slices. Serve immediately.