Kale and Caramelized Onion Grilled Cheese (Adult Grilled Cheese)

4 medium kale leaves, stems removed
2 teaspoons olive or canola oil
2 medium red onions, cut into 1/2-inch-thick slices
1/4 teaspoon black pepper
1/8 teaspoon salt
1 teaspoon red wine vinegar
8 slices pumpernickel, whole wheat or sour dough bread
Cooking spray
1 ounce (about 1/4 cup) grated Parmesan cheese, divided
3.5 ounces (about 1 cup) shredded Mozzarella cheese

Preheat oven to 300°.
Bring a small pot of water to a boil; add kale. Remove from heat; let stand 4 minutes or until kale is bright green.
Meanwhile, heat a skillet over medium-high heat. Add oil, onion, pepper, and salt. Cook 10 minutes or until onion is caramelized, stirring frequently. Remove from heat; stir in vinegar, tossing to coat.
Heat a large nonstick skillet over medium heat. Lightly coat 1 side of each bread slice with cooking spray. Working with 2 slices at a time, arrange bread in pan, sprayed side down. Cook 1 1/2 minutes or until bread begins to brown. Sprinkle 1 tablespoon Parmesan on 1 bread slice in pan. Top with 1 kale leaf, one-fourth of onion mixture, and about 1/4 cup Mozzarella. Top with other toasted bread slice. Transfer sandwich to a baking sheet. Repeat procedure with remaining 6 bread slices, Parmesan, 3 kale leaves, onion mixture, and Mozzarella. Bake sandwiches at 300° for 5 minutes or until cheese melts.

source: slightly modified from the recipe at MyRecipes.com