Heat the chicken patty according to the directions on the package, and allow it to cool while you build the rest of the salad.
To make the dressing: whisk together the mayo, agave, mustard, and pepper until the dressing is smooth.
Add the spinach to a large bowl and top with the tomato, onion, mushrooms, and avocado. Slice the chicken patty into strips and add them to the salad. Top the salad with the honey mustard dressing just before serving.