Add onion, garlic, tomato, avocado flesh, lemon juice, salt, and mustard to a food processor. Blend until smooth, or to your desired texture. (Note: if you peel and de-seed the tomato, it will make for a smoother spread).

Cut cucumber in half lengthwise, and scoop out the seeds; cut into thin slices.

Toast bread, then add a generous helping of avocado spread to each slice; add cucumbers, cheese, and spinach, the close sandwiches and slice into halves or quarters.

Serve with apple slices on the side.