Preheat your grill to medium-high heat.
In a small bowl, mix the garlic powder, onion powder, smoked paprika, salt, and pepper. Rub this spice mixture evenly over the chicken breasts.
Drizzle olive oil over the chicken breasts and ensure they are well-coated.
Place the chicken on the grill and cook for about 6-7 minutes on each side, or until the internal temperature reaches 165°F. Remove from the grill and let rest for 5 minutes.
While the chicken is resting, toast the slices of whole grain bread on the grill until golden brown.
In a small bowl, combine mayonnaise and lemon juice. Spread this mixture on one side of each slice of toasted bread.
Slice the grilled chicken into strips and layer them on two slices of the bread. Top with avocado slices, fresh spinach, and tomato slices.
Place the other slice of bread on top to complete the sandwich. Cut in half and serve.