For the dressing: Whisk together the lemon juice, mustard, honey, salt and pepper until completely blended. Add oil in a slow steady stream, whisking continuously until thick and creamy. Add dill and stir to combine. pour into a storage container and place in the Fridge until ready for use. 

Prep: Cook barley in a medium pot of boiling salted water until tender, 20–30 minutes for pearl barley. Drain; spread out on a baking sheet and let cool. 

Heat olive oil in a large skillet over medium-high until hot. Arrange mushrooms in skillet in a single layer and cook,until golden brown, about 3 minutes. Season mushrooms with salt and pepper, toss, and continue to cook, tossing often and reducing heat as needed to avoid burning. Add onions to the skillet- stir. Reduce heat to medium and add thyme sprigs, garlic, and butter to skillet. Add the asparagus and cook for about 3 minutes- until the butter smells nutty. Turn off the heat and remove mixture to a large bowl.

Assemble the salad: Toss together the Barley,asparagus and mushroom mixture. Add 3 Tablespoon of the dressing ( or more if desired). Serve at room temp. or warm.