Heat the oil in a large skillet or Dutch oven over medium heat. Add the onion and garlic; cook and stir until fragrant. Add the cubed pork, and cook until browned on the outside. Transfer the pork, onions, and garlic to a slow cooker, and stir in the green salsa, jalapeno peppers, and tomatoes.
Cover, and cook on High for 3 hours. Reduce the setting to Low, and cook for 4 to 5 more hours.