Place the onions and capsicum in a bowl along with half of the oil and half of the fajita spice mix. Toss to coat the vegetables. Arrange them in a single layer on the grill tray.
In the same bowl, place the chicken tenderloins and add the remaining oil and fajita spice mix. Stir to coat the chicken.
Place the chicken on top of the vegetables in the grill tray, positioning it in the upper tray position. Press the Combination button once, then turn the dial to select Combi 2. Press the dial to confirm. Set the cooking time to 15 minutes by turning the dial and press the dial to start.
While the chicken is cooking, prepare the guacamole and salsa. For the guacamole, mash the avocado and lime juice in a bowl. Season with salt and stir in the chopped coriander. For the salsa, combine the jalapeno and tomato, and season with salt and pepper.
To serve, sprinkle coriander leaves over the tray bake, and present everything in the middle of the table. Provide sour cream and warmed tortillas for everyone to help themselves.