Heat chicken to 160Ā°F (71Ā°C) internal temperature.
Use melted butter on each bun and grill buns.
Build the sandwich with a bottom bun, 1.5 tbsp of tomato jam, Ā¼ cup of shredded lettuce, chicken breast, grilled onions, and 1.5 tbsp chimichurri sauce, then the top bun.