In a large bowl, whisk together the yogurt, lemon, oil, soy sauce, salt, and 2 cloves garlic (grated or crushed). Add the fish, celery, herbs, and avocado and toss to combine.
In a medium bowl, add the spinach, cheese, and grate the remaining 1 clove garlic into the bowl. Use your hands to toss well so everything gets mixed evenly.
Add the toasted bread to a baking sheet (no parchment). Scoop the salad onto the bread and top with tomato slices (if using). Add the spinach/cheese mixture on top.
Turn your oven to broil on high. Broil for about 2 minutes (keep an eye on it!), or until the cheese is melted, bubbling, and just beginning to get golden brown spots on top.
Cut in half with a serrated knife. I like to eat these with my hands but you could also go knife and fork.