Preheat the oven to 400 degrees F.
Bend the asparagus near the bottom of the stalks to break off the dried ends at a natural breaking point. Discard the ends and cut remaining stalks on an angle into 2-inch pieces. Drizzle with 1 tablespoon of the olive oil, sprinkle with the coarse sea salt, and spread on a baking sheet. Roast until the asparagus is tender and lightly browned, 20 minutes. Remove from the oven and set aside.
Trim the spring onions and slice the white bulbs and light green stems into thin rounds (discard the dark green stems). Drizzle the remaining 1 tablespoon olive oil in a Dutch oven over medium-high heat and add the sliced onions and garlic. Sauté until the onions start to soften, about 2 minutes. Rinse the quinoa, add to the mixture, and toast 2 minutes to lightly toast. Add the stock and mirin and bring to boil. Reduce the heat and simmer, covered, until the quinoa is tender, about 20 minutes. Season with salt and pepper, keeping in mind that the soup will get saltier from the roasted asparagus. Top each serving with a scoopful of roasted asparagus and serve.