Preheat oven to 400°F.
Bend asparagus near bottom of stalks to break off dried ends at natural breaking point. Discard ends and cut remaining stalks on an angle into 2-inch pieces. Drizzle with 1 tablespoon olive oil, sprinkle with coarse sea salt and spread on baking sheet. Roast 20 minutes or until asparagus is tender and lightly browned. Remove from oven and set aside.
Trim spring onions and slice white bulbs and light green stems into thin rounds (discard dark green stems). Drizzle remaining 1 tablespoon olive oil in Dutch oven over medium-high heat and add sliced onions and garlic. Sauté until onions start to soften (about 2 minutes). Rinse quinoa, add to mixture and toast 2 minutes to lightly toast. Add stock and mirin and bring to boil. Reduce heat and simmer covered until quinoa is tender (about 20 minutes). Season to taste with salt and pepper, keeping in mind that soup will get saltier from roasted asparagus. Top each serving with a scoopful of roasted asparagus and serve.
Note: Spring onions can be hard to find, not because they’re not available, but because grocery stores tend to label them inconsistently. Look for slightly overgrown scallions with a rounder white bulb.