Marinate the chicken breasts in olive oil, salt, pepper, and lemon juice for 30 minutes.
Preheat the grill or oven to medium-high heat.
Cook the marinated chicken for 6-7 minutes on each side, or until fully cooked through (165°F internal temperature).
Allow the chicken to rest for 5 minutes before slicing.
Slice the avocado and season with a pinch of salt and a dash of lemon juice.
Toast the whole grain bread until golden brown.
Assemble the sandwich by layering spinach, sliced chicken, avocado, and tomato on one slice of bread, then topping it with another slice.
Slice the sandwich in half and serve immediately.