Chicken and Mushroom Spinach Salad

2 chicken breasts
1/2 c bottled Asian sesame dressing
1 pkg baby Portobello mushrooms, quartered
1 small onion, diced
2/3 c grape tomatoes, halved
1 bag baby spinach

Pound the chicken to about an inch thick. Marinate in the dressing at least an hour in the fridge. Saute or grill the mushrooms and onions until softened ... Char if possible. Grill or broil the chicken just until done, retaining the juices if you can, then slice into bitesized pieces. Toss the veggies, the chicken and juices, and the spinach. Serve with more dressing.