While your grill preheats, prepare all vegetables: scrub the potatoes clean and cut them into 1-inch chunks, slice the carrots into 1/2-inch thick rounds, and chop the onion into 1-inch pieces. In a small bowl, combine the Lawry's seasoning salt, garlic powder, and black pepper. I find mixing the seasonings together first ensures they distribute evenly throughout the vegetables rather than clumping in one spot.

Tear off two large sheets of heavy-duty aluminum foil (about 18 inches each) and arrange them in an X shape on a flat surface, with the center creating a natural bowl. This double-layer approach prevents the foil from tearing during grilling and ensures even heat distribution. Layer all the prepared vegetables into the center of the foil packet.

Scatter the butter cubes evenly over the vegetables, then sprinkle the seasoning mixture from Step 1 over everything. Gently toss the vegetables together to coat them evenly with butter and seasonings. Fold the foil sheets up and over the vegetables, starting with one corner, then the opposite corner, then the two remaining sides, sealing the edges tightly by folding them over themselves—you want a sealed packet that will trap steam during cooking.

Preheat your grill to 350°F. Once hot, carefully place the sealed foil packet directly on the grill grates. Grill for 45 minutes until the potatoes are fork-tender and the carrots are soft. I like to rotate the packet halfway through cooking to ensure even heat exposure on all sides, and I listen for the gentle sizzle inside—that tells you the steam is doing its job.

Remove the foil packet from the grill using tongs and carefully open it (watch out for the steam!). Let the vegetables rest for 2-3 minutes to allow the butter to set slightly. Carefully transfer to a serving dish, pouring any accumulated butter and juices over the top.