1. Preheat oven to 400 degrees F. Have ready 1 large or 2 small baking sheets. Fold four 20-inch pieces of parchment paper or foil in half lengthwise, then open them. Divide asparagus among the 4 pieces of parchment on one side of the fold; there should be at least a 1-inch border at the edge of the paper.
2. Season salmon fillets with salt and pepper on both sides, then place on top of the asparagus. Spread mustard over the surface of the fish.
3. Core the tomatoes and slice off the tops and bottoms; set these pieces aside for another use. Cut each tomato into 4 round slices. Place 2 tomato slices on top of each fillet. Divide mushrooms among the 4 packets. Top with scallions, then top each with 2 thyme sprigs. Fold the edges of parchment or foil together, sealing the edges with tight quarter-inch folds.
4. Place fish packets on baking sheet and cook until fish is cooked through, about 18 to 20 minutes for foil and 22 to 24 minutes for parchment. Let rest 5 minutes before opening the packets. Place each packet on a plate with a lemon wedge if desired, and serve.
Source: Hannaford fresh Magazine, May - June 2013