Pat the pork shoulder dry with paper towels and season generously with salt and pepper. Heat vegetable oil in a large skillet over medium-high heat. Sear the pork on all sides until a deep golden-brown crust forms, about 3-4 minutes per side.

Transfer the seared pork to the basin of a 6-quart or larger slow cooker. Add the chicken broth, soy sauce, quartered onion, smashed garlic, sliced ginger, and shiitake mushrooms.

Cover and cook on low for 8-10 hours or on high for 4-5 hours, until the pork is exceptionally tender and easily falls apart.

Carefully remove the pork from the slow cooker to a large bowl or cutting board. Using two forks, shred the meat, discarding any undesirable fatty pieces. Cover to keep warm.

Strain the broth through a fine-mesh sieve into a large pot or bowl to remove all the solids. Discard the solids. Return the strained, clear broth to the slow cooker and season with additional salt and pepper to taste. Add the shredded pork back into the broth.

When ready to serve, cook the ramen noodles in a separate pot of boiling water according to the package instructions. Drain thoroughly.

Divide the cooked noodles among serving bowls. Ladle the hot broth and shredded pork over the noodles. Garnish with your favorite toppings, such as soft-boiled eggs, sliced green onions, and nori.