Thinly slice half of the shiitake mushroom caps and set aside. Finely chop remaining mushroom caps.
Fill a bowl with ice water. Pick out 6 of the thickest asparagus spears, and using a vegetable peeler, peel long ribbons from each stalk until you’re left with thin planks that are too awkward to slice. Immediately add the ribbons to the ice water. Cut remaining asparagus (including the leftover planks) into 1-inch lengths. Set these cut pieces aside for cooking.
Thinly slice the onion or shallots.
In a medium pan on medium-high heat, add 1 tablespoon oil and the mushrooms. Cook until they turn golden at the edges. Add the remaining 2 tablespoons oil, along with the onions, garlic and ginger, and cook for 1 minute, until fragrant. Add soy sauce and red-pepper flakes. Reduce heat to medium-low and cook for 5 minutes, until reduced.
Increase the heat to medium-high and add asparagus pieces to the pan. Add 1 tablespoon of rice vinegar and the honey. Let cook until the asparagus is crisp-tender, 3 to 5 minutes, adding a tablespoon or two of water to the pan if it dries out. Taste and add salt if needed, and remove from heat.
Drain the asparagus ribbons and pat or spin dry in a salad spinner. Add to a bowl and toss with remaining 1 teaspoon rice vinegar, the sesame seeds, sesame oil and salt to taste.
Divide the grains across four bowls, and top with some of the cooked mushroom and asparagus. Add the asparagus ribbon salad. Drizzle with more soy sauce and sesame oil if needed.