Potatoes a la Ketchup

2 tablespoons ketchup
1 tablespoon Worcestershire sauce
1 tablespoon fresh lemon juice
4 tablespoons (1/2 stick) unsalted butter, at room temperature
4 medium potatoes, cut into 1/4-inch slices
1 small onion, thinly sliced
Salt and freshly ground pepper, to taste

Set up grill for indirect grilling and preheat to medium.

Mix the ketchup, Worcestershire sauce and lemon juice in a small bowl and set aside.

Cut 4 sheets of heavy-duty aluminum foil, each 14 by 18 inches. Place a sheet of foil, shiny side down, narrow edge toward you, on the work surface. Smear a tablespoon of the butter in the center of the bottom half of the rectangle (the half closest to you). Arrange one-fourth of the potatoes in a mound on top of the butter. Place one-fourth of the onion on top of the potatoes.

Spoon 1 tablespoon of the ketchup mixture over the potatoes and season with salt and pepper. Fold the top half of the foil over the potatoes and bring the top and bottom edges together. Fold the edges over several times to make a tight seal. Prepare the remaining packages the same way.

When ready to cook, place the foil packages in the center of the hot grate, away from the heat. Cover the grill and cook until the packages are dramatically puffed, 20 to 30 minutes.

Serve, warning everyone to open the packages at arm's length, using knives and forks, as the escaping steam will be very hot.