Prepare the slow cooker by coating with a light coating of cooking spray.
Pat the skirt steak dry with paper towels. Season both sides with salt, pepper, paprika, garlic powder, and onion powder.
Heat olive oil in a large skillet over medium-high heat. Sear the steak for 2–3 minutes per side until browned.
Place the sliced onion and minced garlic in the bottom of the slow cooker.
Lay the seared steak directly on top of the onions and garlic.
In a small bowl, whisk together beef broth, tomato paste, Worcestershire sauce, and balsamic vinegar until smooth.
Pour the sauce evenly over the steak.
Cover and cook on low for 6–8 hours or high for 3–4 hours, until the steak is tender and infused with flavor.
Remove the steak and let it rest for 5–10 minutes before slicing thinly across the grain.
Serve with the onions and spoon over the savory cooking liquid.