In a small bowl, combine the softened butter, minced garlic, chopped parsley, chopped dill, fresh lemon juice, lemon zest, salt, and pepper. Mix until well combined to form the garlic herb butter.
Cut four sheets of aluminum foil (about 12 inches long) large enough to wrap each salmon fillet.
Place a salmon fillet in the center of each piece of foil. Spoon a generous amount of the garlic herb butter over each fillet.
Top each salmon fillet with a few slices of lemon and a sprig of thyme, if using.
Fold the edges of the foil up to create a tightly sealed packet, ensuring that no juices can escape.
Place the foil packets on a baking sheet and bake in the preheated oven for 15-20 minutes, or until the salmon is cooked through and flakes easily with a fork.
Carefully open the foil packets (watch out for steam!) and serve the salmon directly from the foil or transfer to plates.