Heat oven to 425°F. Coat an 18x13-inch rimmed baking sheet with a little additional oil.
On a rimmed baking sheet mix together sliced onions, chicken, and 1/4 - 1/3 cup dressing until onions and chicken are well coated. Roast in preheated oven for 20 minutes.
In a medium bowl marinate mushrooms with 2-3 tablespoons of dressing. In a second bowl, marinate strawberries with 3-4 tablespoons of dressing. There will be dressing leftover.
When chicken/onion mixture has roasted for 20 minutes, top with marinated mushrooms and cook for an additional 8-10 minutes. Chicken is ready when its juices run clear when cut into the thickest part, or its internal temperature reads at least 165°F with an instant read thermometer. When ready remove baking sheet from oven and let cool for 2-4 minutes.
Place spinach on plates or serving platter. Top with chicken/onion/mushroom mixture then marinated strawberries and optional toasted almonds. If desired, add a little additional Italian Balsamic dressing and serve.