Blend sauce. Combine adobo paste, tomatillos, garlic, lime juice, salt, and 1/4 cup chicken broth in a blender. Blend on high until smooth.

Season beef. Generously season beef on all sides with salt and pepper.

Brown beef. Heat oil in a large frying pan over medium-high heat. Once shimmering add beef and brown about 4 minutes a side (you might need to do this in batches depending on the size of your pan).

Add veggies to slow cooker. Meanwhile, place onions and jalapeños on the bottom of the slow cooker.

Deglaze pan. Once beef is browned transfer to the slow cooker. Pour off any fat from the frying pan into a heatproof bowl. Return pan to heat and add 1 cup of the broth to the frying pan. Scrape any browned bits off the bottom of the pan with a wooden spoon, then pour broth into the slow cooker.

Combine in slow cooker and cook. Add adobo mixture and remaining 1 cup of broth to the slow cooker, cover, and cook on low for 8 hours or high for 4.

Serve! Beef should be fall-apart tender when done. Shred meat and serve as tacos, enchilada filling, on top of a rice bowl, or straight from the pot with a fork. Add more salt as needed.