4large thick slices of sourdough or whole-grain bread
Instructions
Heat the oil in a large skillet over medium heat. Add the sliced red onion and saute for 5 to 7 minutes, stirring occasionally. The onions should be soft and lightly caramelized. If they start to crisp too quickly and get dry, add a tablespoon of water to deglaze the pan.
Add the sliced broccolini to the skillet and continue to sauté over medium heat for an additional 3 to 4 minutes, or until the broccolini is fork tender. Add the red pepper flakes and stir to combine. Season to taste with salt and place mixture in a heatproof bowl and set aside. Clean and dry the skillet.
Combine the grated cheeses together in a separate bowl. To assemble the sandwiches, spread butter (roughly ½ tablespoon) evenly onto the outside of each bread slice. Add a layer of cheese onto the bottom piece of bottom. Top with a layer of the vegetable mixture, and then add another handful of cheese. Add the top layer of bread.
Heat the same skillet to medium-low heat. Cook the sandwiches until golden brown on each side, adjusting the heat as necessary. Slice sandwiches in half and serve immediately.