Preheat the oven to 425 degrees. Line a large baking sheet with parchment paper.
Trim off and discard the dried, tough part of the broccoli stem. Working from the stem end, slice the broccoli lengthwise into 4 slabs.
Arrange the broccoli slabs in a single layer and brush on all slides with olive oil and season with garlic powder, salt, and pepper. Roast for 10 minutes, flip, and roast another 10minutes.
Meanwhile, spread a thin layer of marinara sauce on the bottom pieces of the ciabatta rolls. Spread more marinara on the top piece of the roll, making a thicker layer of sauce. Once broccoli is done roasting, place the broccoli slab on top of that bottom piece and top with mozzarella slices to cover. Place all of the ciabatta pieces in the oven and cook for 5 to 8 minutes or until mozzarella melts and top slices are warmed. Garnish melted mozzarella with Parmesan cheese, red pepper flakes, and basil.
Once cheese is melted, close the sandwiches and serve.