1. Cut the chicken in 1 inch cubes. Heat the oil in a large pan and add the chicken. Season with salt and pepper, and saute until chicken is cooked all the way through, about 6-8 minutes, or until juices run clear. Set aside, keep warm.
2. Add the garlic to the pan and stir until aromatic, around two minutes. Add the drained diced tomatoes, mushrooms and spinach. Use tongs to toss in the pan. Cook 3-4 minutes, until the spinach is wilted and most liquid has been absorbed. Adjust the seasoning if necessary.
3. Add the chicken back to the pan and toss.