01 - Heat a grill or grill pan to medium-high temperature.
02 - Place chicken breasts between plastic wrap sheets and pound to even 3/4-inch thickness for uniform cooking.
03 - Brush chicken with olive oil, then evenly sprinkle with garlic powder, onion powder, dried thyme, salt, and black pepper.
04 - Grill chicken for 5 to 7 minutes per side until golden brown and internal temperature reaches 165°F.
05 - In the final 2 minutes of grilling, top each chicken breast with a slice of ham and Swiss cheese. Cover grill or tent with foil until cheese melts.
06 - In a mixing bowl, blend mayonnaise, Dijon mustard, honey, and lemon juice until smooth.
07 - Lightly butter the cut sides of buns and toast them on the grill, cut-side down, until golden if desired.
08 - Spread Dijon sauce over bottom halves of buns, layer with baby spinach or lettuce, grilled chicken with ham and cheese, and cover with top buns.
09 - Serve sandwiches immediately while hot and cheese is melted.