Pork Tenderloin and Collards Skillet

unsalted butter, divided

pork tenderloin, trimmed

kosher salt, divided

black pepper, divided

sliced yellow onion

chopped red bell pepper

garlic cloves, sliced

fresh thyme leaves

thinly sliced stemmed collard greens

canned unsalted black-eyed peas, rinsed and drained

apple cider vinegar