Preheat the oven to 425 degrees. Cut a piece of parchment paper to fit a shallow baking pan.
Wash the sweet potatoes. Slice the potatoes and onions 1/4-inch thick.Put sweet potatoes and onions on the baking sheet in a single layer.Drizzle with the olive oil and sprinkle with the salt. Bake for 15 minutes.
Meanwhile, mix the garlic,mustard, lemon juice, and rosemary to make a paste and set aside.
Remove the sweet potatoes and onions from oven (keep in the baking pan). Place the asparagus on the paper around the sweet potatoes and onions. Sprinkle the lemon zest on the asparagus. Lay the salmon on top of the asparagus and onions. Spread the paste on top of the salmon.
Return the pan to oven and roast for 12 minutes. Salmon is done when the flesh flakes with gentle pressure.