Preheat the oven to 350°F.
In a large pot, place 1 inch of water, a steamer basket and the potatoes. Cover and bring to a simmer. Steam the potatoes until fork-tender, 15-20 minutes. Drain and return to the pot. Gently toss the potatoes with the oil, salt, pepper, lemon juice and parsley.
Combine all of the dill honey mustard ingredients until smooth. Use a blender for a perfectly smooth sauce if you like. Season to taste.
Pat the salmon fillets dry and coat with half of the honey mustard. Reserve the remaining mustard for serving (be careful not to contaminate it with raw fish). Roast the salmon for 15-20 minutes, until cooked through.
Bring a large pot of salted water to a boil for the asparagus. Prepare an ice bath by filling a large bowl with water and ice. When the water boils, add the asparagus for 2-4 minutes then immediately transfer to the ice bath to stop cooking. Remove from the ice bath after 1-2 minutes.
Add the asparagus, spinach and red onion to the pot with the potatoes and toss gently. Divide the vegetables between plates, top with salmon fillets and drizzle with reserved honey-mustard.