1½ cups thinly sliced yellow onion (approximately 2 small onions)
½ teaspoon fine sea salt
3 tablespoons water
6 slices pumpernickel rye bread
6 tablespoons mayonnaise
12 (¼-inch-thick) slices Brie cheese
¾ cup spinach and baby kale mix
Garnish: baby kale leaves
Instructions
In a small sauté pan, melt butter over medium heat. Add onions and salt; cook, stirring occasionally, until caramelized, 20 to 25 minutes. Add water, stirring to deglaze pan and remove any stuck-on pieces of onion.
Spread each bread slice with an even layer of mayonnaise. Arrange cheese in a single layer on mayonnaise side of 3 bread slices. Top with caramelized onions, and let cool slightly. Top with spinach and kale mix and remaining bread slices, mayonnaise side down, to make 3 whole sandwiches.
Using a serrated bread knife in a gentle sawing motion, evenly trim and discard crusts from sandwiches. Cut each rectangular sandwich into 6 (approximately 2¼-inch) squares. Serve immediately, or cover with damp paper towels, place in an airtight container, and serve within an hour.
Just before serving, garnish with baby kale leaves, if desired.
Notes
MAKE-AHEAD TIP: Onions can be caramelized up to a day in advance, placed in a covered container, and refrigerated. Warm before using.
Recipe by TeaTime Magazine at https://teatimemagazine.com/spinach-kale-caramelized-onion-brie-tea-sandwiches/