Heat oil in a large skillet over high. Cook pork 3 minutes per side, until slightly brown. Transfer to a slow cooker coated with cooking spray. Lower heat to medium and add onion, cut side down. Cook ~5 minutes, until charred. Transfer to slow cooker.
Add broth, water, soy sauce and Hoisin to slow cooker. Cover and cook on LOW for 7 hours.
Remove pork from slow cooker and place on a cutting board. Let rest for 5 minutes before using 2 forks to shred. Discard excess fat.
Strain liquid from slow cooker through a fine sieve into a bowl. Discard all solids (including onions). Return liquid to slow cooker.
Increase heat to HIGH. Add mushrooms, ginger, garlic and lemongrass to slow cooker, cover and cook for 30 minutes.
Add noodles and shredded pork to slow cooker and cook, covered, for 5 minutes.
To assemble: ladle ramen into bowls and top with soft boiled eggs, cilantro, sliced green onion, a squeeze of lime juice and sprinkle of sesame seeds.