3 pounds beef chuck, trimmed and cut into 1-inch cubes
1 tablespoon avocado oil or grapeseed oil
1 large onion, halved and sliced
1 cup low-sodium beef broth
3 tablespoons tomato paste
1 cup chopped fresh rhubarb
1 teaspoon white sugar
½ cup chopped yellow bell pepper
1-2 jalapeños, seeded if desired, finely chopped
¼ cup finely chopped red onion
¼ cup chopped fresh cilantro
1 tablespoon lime juice
16 corn tortilla
Lime wedges for serving
Combine chili powder, oregano, cumin, garlic powder and 1¼ teaspoons salt in a small bowl. Sprinkle beef with 2 tablespoons of the mixture.
Heat oil in a large skillet over medium-high heat. Add the beef and cook, stirring occasionally, until browned on all sides, 5 to 6 minutes total. Place onion slices in a 5- to 6-quart slow cooker. Add the beef. Whisk broth and tomato paste into the remaining spice mixture and pour over the beef.
Cover and cook on High for 4 hours or Low for 8 hours. Shred the beef with 2 forks. Stir back into the liquid.
Meanwhile, toss rhubarb and sugar together in a medium bowl. Let sit for 10 minutes. Add bell pepper, jalapeño to taste, red onion, cilantro, lime juice and remaining ¼ teaspoon salt and stir to combine.
Serve the beef with tortillas, the rhubarb salsa and lime wedges.