Wash the fresh fish and pat dry. (If using canned herring, drain completely.) Cut fish into strips or small cubes and sprinkle with lemon juice.
Wash zucchini and tomatoes and cut into thin slices. Rub peeled garlic onto the Zwieback toasts, and spread the toasts with butter. Lay fish pieces, zucchini and tomato slices on top, and season with salt and pepper. Cover with sauce Béarnaise and slivers of almonds and grill at high heat for about 5 minutes.