Place pork, onions, salt, pepper, cumin, and bay leaves into a large braising pan; cover with water; gently simmer over medium-heat 1 1/2 hours or until pork is tender; drain liquid from pork; remove bay leaves; trim pork into 1/2" pieces.
NOTE: A portion of the pork may be cooled and frozen at this point, then used for making carnitas later on.
Wipe braising pan dry; pour in vegetable oil and heat until oil almost smokes; brown pork in oil, stirring occasionally, for approximately 15 minutes.
While pork is still cooking, prepare the sauce: Remove most of the membranes and most of the seeds from the jalapeno peppers; bring chicken stock to boil in medium saucepan; add tomatillos, jalapenos, and garlic cloves and boil 3 minutes; pour hot contents of saucepan into blender; puree.
Pour sauce over browned pork; simmer 15 minutes over low heat; thicken, if desired, with a slurry of 3 tablespoons masa harina shaken with 1/4 cup water until smooth.
NOTE: You may make your chile verde as spicy or as mild as you like by simply increasing or decreasing the amount of jalapeno pepper seeds and membranes included in the sauce.