Add onion and cook, stirring frequently, until golden brown, 5 to 7 minutes.
Add vinegar and cook until almost evaporated, about 1 minute.
Add garlic and cook, stirring, until fragrant, about 30 seconds.
Add kale, water and salt (the pan will be full).
To prepare panini: Preheat panini maker to high.
Coat one side of each slice of bread with cooking spray.
Top each with 1/4 cup cheese and 2 slices tomato.
Top with the remaining bread, sprayed-side up.
Press in the panini maker until crispy, 3 to 5 minutes.
Place 2 sandwiches in the pan.
Place a medium skillet on top and add four 15-ounce cans to weight it down.
Cook the sandwiches, turning once, until golden brown, 2 to 3 minutes per side.
Repeat with the remaining sandwiches.
- Daily Values (DVs) are the recommended amounts of nutrients to consume each day.
(-) Information is not currently available for this nutrient.
