Place two large sheet pans in the oven and preheat oven to 400 degrees.
In a medium bowl, toss the potatoes with 2 tablespoons olive oil, thyme, and 2 teaspoons salt. Once the oven is to temperature, divide the potatoes among the sheet pans to roast while you prepare the remaining ingredients.
In a small bowl, use a fork to whisk together the remaining 1 tablespoon olive oil, Dijon mustard, and honey together.
After the potatoes have cooked for 20 minutes, remove the pans from the oven and stir the potatoes.
Add the salmon fillets and the asparagus to the sheet pans and spoon 1 teaspoon of the honey mustard on top of each salmon fillet and brush to coat. Reserve the remaining honey mustard for later.
Drizzle the asparagus with the remaining 1 tablespoon olive oil and season the entire sheet pan with the remaining 1 teaspoon salt and 1 teaspoon pepper.
Place the pan in the oven and roast until the asparagus and potatoes are tender and salmon is cooked through, about 20 minutes.
While the salmon cooks, add the mayonnaise and whole-grain mustard to the honey mustard and stir to combine. Store in the refrigerator until ready to eat.
Serve immediately with honey mustard sauce on the side.