Place the tuna in a medium bowl and break up with a fork until finely shredded.
Add the carrot, celery, onion, lemon zest, juice, parsley and mayonnaise to the tuna. Stir to combine thoroughly.
Season with salt and pepper, to taste.
Toast the bread for 2-3 minutes.
Divide the tuna between each slice of toast and pile in an even mound.
Top each sandwich with one tomato slice and sprinkle with 1/4 cup cheese. Repeat with the remaining sandwiches.
Arrange the sandwiches on a baking sheet and broil on high for 1-2 minutes until the cheese is melted.
Top with the sprouts or micro greens, if desired and serve immediately.
Notes
The Tuna Salad can be made up to 3 days in advance and stored in an airtight container. It's also amazing stuffed in an avocado or a hollowed out tomato!