Preheat the oven to 375°F (190°C).
Season the chicken: sprinkle the 4 chicken breasts with 1 teaspoon kosher salt, 1/2 teaspoon black pepper, 1/2 teaspoon paprika, and 1 teaspoon Italian seasoning; set aside.
Combine the potatoes, sliced onion, minced garlic, olive oil, and the remaining seasonings (the remaining 1 teaspoon Italian seasoning and remaining salt/pepper/paprika) in a large bowl and toss to coat evenly.
Prepare four foil sheets and spray the center of each with nonstick spray. Divide the potato mixture evenly among the four sheets, placing it in the center of each.
Top each pile of potatoes with one seasoned chicken breast, a few lemon slices, and 1 tablespoon unsalted butter.
Fold the long sides of the foil toward the center, roll the edges down to seal, then fold the short ends to create a tight packet. Repeat for all packets.
Place the foil packets on a baking sheet and bake in the preheated oven for 35 minutes, or until the chicken reaches 165°F and the potatoes are tender.
Let the packets rest sealed for 5 minutes, then carefully open and garnish with fresh herbs if desired before serving.