Prepare the marinade by combining the ingredients in a jar. Shake or stir to combine.
Add the chicken to a large bowl or bag and pour half of the marinade over the top. Stir to combine and coat all of the chicken. Cover and place in the refrigerator to marinate for at least 30 minutes, 2-4 hours for best flavor (over-marinating can lead to mushy chicken).
Reserve the other half of the marinade to use as a salad dressing.
Prepare the vegetables for grilling. Slice the onion into rounds, lightly brush both sides with oil and sprinkle with salt. Lightly brush onto all sides of the peppers. Remove the husks and silk from the corn, lightly brush oil onto the corn and sprinkle with salt.
Preheat grill to 400-425 degrees F, set the entire grill to cook direct heat.
Remove the chicken from the marinade, pat dry.
Rub a small amount of oil on the grill grates.
Place the chicken directly on the grill. Then add the vegetables. Close the lid and cook for 6-7 minutes. Then open the grill, flip the chicken and turn all of the vegetables. Close the lid and continue cooking for another 5-8 minutes until chicken registers an internal temperature of 180 degrees F (165 degrees F if cooking chicken breasts), and vegetables are charred and softened.
Remove the chicken and vegetables from the grill and place on a large baking sheet. Let the chicken and poblanos rest for at least 5 minutes before cutting.
Once the poblanos have cooled to the touch, place on a cutting board. Peel the skin, it should come off with your fingers, use a paring knife if needed to scrape off any stuck on skin. Slice off the stem, then down the middle, removing the seeds from the inside. Chop the peppers into 1 inch pieces.
Cut the kernels off the corn.
Slice the grilled chicken into thin strips.
Prepare the salad by layering the romaine lettuce in a salad bowl or dish.
Top with the chicken, poblanos, corn, grilled onion rounds, sliced tomatoes, avocado, top with crumbled queso fresco.
Serve with the reserved dressing.