Spray the liner of a 6 quart or similar size crockpot with cooking spray.
Use a sharp knife to trim fat and silver skin from each tenderloin. Pat dry with a paper towel. Drizzle on all sides with olive oil. (eyeball the amount)
In a small bowl whisk together brown sugar, chili powder, salt, lemon pepper, paprika, granulated garlic, onion powder and cumin.
Rub both tenderloin with seasoning rub. In a large skillet over high heat with a light drizzle of olive oil, sear the seasoning onto the tenderloin on all sides.
Arrange the pork side by side in the slow cooker. Tuck the onion wedges between and around the sides of the pork.
Drizzle with 1 1/2 cups barbecue sauce. Lock the lid firmly in place.
Cook on low for 2 1/2 hours OR until the pork reaches 145°F.
Remove the pork to a cutting board and let stand uncovered for 5 minutes while you thicken the sauce.
Thicken the Sauce: Increase the crockpot temperature to high. Whisk tomato paste into the drippings to thicken. Begin with 2 Tbsp adding more until your desired thickness is reached.
Use a sharp knife to thinly slice the tenderloin. Serve immediately drizzled with sauce.