In a medium sauté pan, melt butter over medium heat. Add garlic; cook until fragrant, approximately 2 minutes. Stir in spinach, 1 teaspoon lemon zest, lemon juice, and salt. Cook, stirring occasionally until spinach wilts, approximately 3 minutes. Drain and discard juices. Let cool completely.
In a small bowl, stir together mascarpone cheese, remaining 1 teaspoon lemon zest, and pepper. Spread mixture onto each bread slice. Top 6 bread slices with an even layer of spinach and a layer of chicken. Top with remaining bread slices, spread side down.
Using a serrated bread knife in a gentle sawing motion, trim and discard crusts from sandwiches. Cut each sandwich into 2 (3×1-inch) finger sandwiches. Cover with damp paper towels, place in an airtight container, and refrigerate until ready to serve.
Garnish with thyme sprigs, if desired.
Recipe by TeaTime Magazine at https://teatimemagazine.com/chicken-spinach-tea-sandwich/