01 - Preheat a grill or grill pan to medium-high heat.
02 - Place chicken breasts between sheets of plastic wrap and pound to an even 0.75-inch thickness for uniform cooking.
03 - Brush chicken with olive oil and evenly season with garlic powder, onion powder, dried thyme, salt, and black pepper.
04 - Grill chicken breasts for 5 to 7 minutes per side until browned and cooked through; internal temperature should reach 165°F.
05 - In the final 2 minutes of grilling, top each chicken breast with a slice of ham, then a slice of Swiss cheese; cover grill or tent with foil to encourage cheese melting.
06 - Whisk together mayonnaise, Dijon mustard, honey, and lemon juice until creamy and well combined.
07 - For extra flavor, lightly butter the sandwich buns and toast them cut-side down on the grill until golden brown.
08 - Spread Dijon sauce on the bottom halves of toasted buns, layer with baby spinach or lettuce, place grilled chicken topped with ham and cheese, then finish with top bun.
09 - Serve sandwiches immediately while hot and cheese is melted.