Season the oxtails generously with sea salt and black pepper on all sides.
Heat the olive oil in a heavy-bottomed pot or Dutch oven over medium-high heat and sear the oxtails until they are deeply browned and caramelized.
Add the diced yellow onion, minced garlic, and tomato paste to the pot, stirring constantly for 2 minutes to release the aromatics.
Pour in the beef broth and add the fresh thyme sprig, ensuring the liquid partially covers the meat.
Reduce the heat to low, cover with a tight-fitting lid, and simmer for 2.5 to 3 hours until the meat is fork-tender and falling off the bone.
While the meat is finishing, combine the cooked quinoa, rinsed black beans, and diced red bell pepper in a medium bowl.
Remove the oxtails from the pot and serve them warm over the refreshing quinoa and bean salad, drizzling with a spoonful of the braising liquid.