To make salsa: Combine white onion, tomatillo, jalapeno and garlic in a blender and puree until very smooth.
Add cilantro and puree until very smooth and vibrant green.
Refrigerate for up to 2 days.
To make pork: Preheat oven to 300°F.
Cut pork butt into 6 equal pieces.
Place all ingredients together in an oven proof baking dish that is deep enough to fit all ingredients.
Make sure that the pork butt is 3/4 of the way covered with all the ingredients.
Cover with lid or aluminum foil.
Cook for 3 hours.
Remove pork from oven after 3 hours.
Check the pork to make sure its tender. It should easily pull apart with a pair of tongs.
Let pork cool down in the braising liquid for an hour.
Remove from braising liquid and pull the pork apart with your hands, tongs or a fork.
To prepare the taco: In a skillet over medium heat 1 pound of the cooked pork, julienne onions and garlic in 1 tablespoon olive oil for about 2 to 3 minutes.
Squeeze the juice of 2 limes and fold in chopped cilantro.
Season with salt.
Heat the tortillas until warm on a skillet or microwave.
Place cooked pork (carnitas) onto the tortilla. Top with salsa verde and serve.