1. Preheat the oven to 425 degrees F.
2. Line a baking sheet with parchment paper. Place the sweet potato and onion slices on the parchment in a single layer. Drizzle with the olive oil and sprinkle with sea salt. Bake for 15 minutes.
3. Meanwhile, mix the garlic, dry mustard, lemon juice, and rosemary to make a paste. Set aside.
4. Remove the baking sheet from the oven, place the asparagus on the parchment next to the sweet potatoes and onions. Sprinkle the lemon zest on the asparagus. Lay the salmon on top of the asparagus and onions. Spread the mustard paste on top of the salmon.
5. Return the sheet to the oven and roast for 12 minutes. The salmon is done when the flesh flakes with gentle pressure. Serve the salmon on top of the asparagus, sweet potatoes, and cooked onions. Any leftovers can be refrigerated and eaten within 3 days.
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