Bring a large pot of water to a boil. Add the oxtail and boil for 5 minutes. Drain, rinse the oxtail well, and pat dry.
Heat the neutral oil in a large Dutch oven or heavy-bottomed pot over medium-high heat. Add the oxtail and sear until deeply browned on all sides, about 8 to 10 minutes.
Add the onion, garlic, ginger, dried shiitake mushrooms, and 8 cups water to the pot. Bring to a gentle simmer.
Reduce the heat to low and simmer gently for 3 1/2 to 4 hours, or until the oxtail is very tender and the broth is rich and lightly reduced. Avoid a hard boil so the broth stays smooth.
Remove the oxtail from the pot. Strain the broth through a fine mesh strainer. Skim off excess fat, leaving a small amount for flavor.
When cool enough to handle, remove the oxtail meat from the bones and shred into bite-size pieces. Discard the bones and aromatics.
In a small bowl, stir together the soy sauce, mirin, and sugar until the sugar dissolves. This is the light shoyu tare.
In a small saucepan, warm the neutral oil or reserved beef fat with the grated garlic over low heat for 1 to 2 minutes, just until fragrant. Do not brown the garlic.
Cook the ramen noodles separately according to package directions. Drain well.
To assemble each bowl, add about 1 tablespoon shoyu tare and 1/2 teaspoon garlic oil to the bottom of the bowl.
Pour in 2 to 2 1/2 cups hot oxtail broth and stir. Taste and add a little more tare if needed.
Add the cooked ramen noodles, shredded oxtail, soft-boiled egg halves, bok choy or spinach, green onions, sesame seeds, and chili oil if using. Serve hot.