TIP: You can also use halibut or other meaty fish in the packets; just be sure to adjust the cooking time.
1 large yellow squash, sliced into 1/4-inch-thick half-moons
1 bunch asparagus, cut into 1-inch pieces on the diagonal
Freshly cracked black pepper
Four 6-ounce white fish fillets, such as turbot, perch, sole, or cod (see Cook’s Note)
1 large lemon, thinly sliced (14 to 16 slices), seeds removed
8 tablespoons (1 stick) unsalted butter
16 sprigs thyme
1/4 cup chopped flat-leaf parsley, for garnish
Lemon wedges, for serving
Preheat your grill on medium-high heat, about 400°F.
Place 4 large sheets of heavy duty or grilling foil on a clean surface. They should roughly measure 15 x 18 inches.
First, divide the squash and asparagus between the sheets of foil. Season with salt and pepper. Next, place the fish on top (cut the fillets to fit if necessary) and season with salt and pepper. Add 3 to 4 lemon slices along with 2 tablespoons of butter to each. Strip the thyme leaves from the sprigs and sprinkle the leaves over the top along with more salt and pepper.
Bring the longer sides of a sheet of the foil up and fold it over several times until the foil is snug against the contents of the packet. Fold the shorter sides inward to make a leak-proof packet. Continue with remaining packets.
Place the foil packets on the grate, close the lid and grill for 10 minutes. Open the grill and rotate the packets, moving the ones closer to the back closer to the front. Cook until the fish is cooked through and the veggies are fork tender but still al dente, 5 to 7 minutes longer.
Remove and open the packages. Sprinkle with parsley and serve with extra lemon wedges.