Cut the flank steak against the grain into ⅛-inch (3-mm) thick slices.
In a large skillet over medium heat, heat the olive oil, until shimmering. Add the steak, paprika, garlic powder, ½ teaspoon of pepper, 1 teaspoon of salt, and the Worcestershire sauce.
Mix to coat the meat with the spices and cook until the meat is cooked through, about 3 minutes. Transfer the meat a bowl, leaving the juices in the pan. Cover and set aside until ready to use.
Add the butter to the pan with the meat juices. Add the onions and cook until tender and shiny, about 10 minutes. Add the green bell peppers and the remaining ½ teaspoon of pepper and teaspoon of salt, and cook until the onions and peppers are caramelized, about 10 minutes.
Cut the top off of a red bell pepper. Cut around the seed pod, then pull it out of the pepper. Remove any remaining seeds. Repeat with the rest of the peppers.
Cut 9 of the provolone slices in half.
Place the peppers in a 9x13-inch (23x33-cm) baking dish, cut side up. Scoop a bit of steak filling into each pepper, followed by the peppers and onions, then 3 half slices of provolone. Repeat with the rest of the steak, then the rest of the peppers and onions. Place a whole slice of provolone on top of each pepper.
Bake for 30 minutes, until the skin of the peppers is shriveling, and the cheese is golden.
Top with parsley.
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