Bring a pot of salted water to a boil. Add the barley and return to a boil. Reduce heat to medium and cook over a vigorous simmer for 30-35 minutes. Check the barley after 25 minutes and cook until the barley is tender yet chewy. Turn off the heat and drain the barley.
Prepare the Broth:
While the barley is cooking, prepare the dashi broth. Heat the oil in a wide pot over medium heat. Add the onion and cook for 5-7 minutes until softened and beginning to brown.
Add the garlic, ginger, and Thai chili pepper and cook for 1 minute until fragrant.
Add the chicken stock and bring to a boil.
Pour in the prepared dashi stock, soy sauce, mirin, and sesame oil. Reduce heat and simmer for 25 minutes. Taste and season with salt, if necessary.
Prepare the Chili Oil Daikon Radish:
Combine the finely diced daikon radish with chili oil, sesame oil, salt, and soy sauce and toss to combine. Set aside.
Cook the Scallops:
Taste the broth and season again to your preferences. Pat the scallops dry and season lightly with salt. Nestle the scallops in the broth. Cover and cook for 5-6 minutes until opaque and cooked through. Turn off the heat and remove the lid.
To Serve:
Divide the cooked barley between shallow bowls and ladle the broth on top. Arrange the scallops on each dish and garnish with the daikon radish, spooning a bit of the chili oil from the radish over the scallops. Arrange scallions or pea shoots on each dish. Enjoy!