Add 2 quarts cold water to a mixing bowl along with 1/2 cup kosher or sea salt. Stir to dissolve. Cut the pork shoulder into 2" chunks. Add the pork chunks to the brine, cover, and store in the fridge for an hour. If the pork isn't completely submerged just add a bit more water.
Once brined, remove the pork chunks and pat them dry. Add them to a clean mixing bowl along with: 2 teaspoons Mexican oregano, 1 teaspoon cumin, 1/2 teaspoon chipotle powder (optional), 1 teaspoon salt, freshly cracked black pepper, and 2-3 tablespoons oil. Combine well.
Add pork pieces to a casserole dish along with the juice of 1 orange, the juice of 1 lime, 1/2 onion cut into chunks, 1 jalapeno sliced in half, and 4 peeled garlic cloves. I usually put the leftover orange pieces in the pan as well. Cover tightly with foil and bake at 300F for three hours.
After three hours give it the fork tender test. If you can insert a fork into the pork with no resistance then it's done. If the chunks are still firm give it another half hour.
Remove the pork and set aside. Strain the resting liquid into a bowl and be sure to give it a taste! As it sits the fat will rise to the top -- you can remove some of the fat if you want but I use most of it.
To crisp up the Carnitas we'll put the shredded pork chunks under the broiler (approx. 525F). You can use the same baking dish, just be sure to remove the leftover fruit and veggie chunks. Add the pork back to the dish and shred with two forks. I usually give it a partial shred, leaving it chunky, but you can always shred finer if you want.
Drizzle some of the resting liquid over the pork and broil for 5 minutes. Take a look and broil for another 5 minutes if you want it crispier.
To make the Salsa Verde, it's best to put the tomatillos in the oven about 20-30 minutes before you think the pork will be done. Pull the husks off the tomatillos , give them a good rinse, and cut the stems out. Roast the tomatillos in the oven for 20 minutes or until they start to turn army green. They'll need a bit more time to roast because the oven temp is lower than usual. Add roasted tomatillos to a blender along with 1/2 onion, 1 garlic clove, 10-12 sprigs cilantro, and 1-2 serrano peppers. Combine well and taste for heat level. For a milder version start with 1/2 serrano pepper and add more to increase the heat.
For serving you'll need corn tortillas, 1/2 cup finely chopped onion, some freshly chopped cilantro, the Salsa Verde, 3-4 limes, and cheese (optional). You can warm up the tortillas in the oven but you'll get happier faces if you crisp them up in hot dry skillet first. I usually put a large skillet over medium high heat and crisp up 3-4 at a time, storing them in a tortilla warmer or tea towel.
If there are Carnitas leftovers, store in an airtight container in the fridge. I usually store the resting liquid separately. To re-heat, simply add pork to a saucepan along with some of the resting liquid, cover, and simmer over medium until heated through.