These foil packet dinners are as great for camping as they are for an easy weeknight meal on the grill.
Preheat grill to medium (350°F to 400°F). Stir together butter, thyme, and garlic in a small bowl until incorporated; chill 10 minutes while packets are prepared.
Cut 4 (18- x 12-in.) sheets of heavy duty aluminum foil. Divide onions among each sheet of foil, placing them in the center. Layer 4 baby potato halves, 5 sweet potato pieces, 4 mini pepper halves, 1/4 pound of sausage, 3 jalapeño slices, and 2 lemon slices on top of each packet of onions. Drizzle each with 1 teaspoon of oil, and sprinkle evenly with salt and pepper. Dollop 1 tablespoon of thyme butter on top of each. Fold long ends of foil together and crimp to secure; fold remaining edges toward center of packet and seal.
Grill packets, uncovered, 25 minutes. Carefully open packets to release steam, and enjoy straight from foil, or transfer to a plate, and drizzle with delicious juices.