Steam carrots and asparagus until tender-crisp, and onions until tender. Immediately submerge in ice water to stop the cooking process. Drain, pat dry and set aside. The dish can be made ahead of time to this point. Store veggies in fridge until ready to serve.
Whisk together the oil, vinegar, mustard, and sugar then pour into a skillet. Once mixture starts to bubble, add veggies, water and herbs and toss to combine and until veggies are warmed throughout. Season to taste with salt and pepper.