Preheat a grill, grill pan, or cast-iron skillet. Season the chicken all over with salt and pepper and grill or sear for 3 to 4 minutes per side, until firm and cooked through. Divide the mixed greens among the bun bottoms. Top each bun with a chicken thigh, then pile on the onion and peppers. Spoon on the chimichurri, then top with the other bun halves.