Too late for last night's dinner, but...
My favorite seasoning for a leftover steak sandwich: Spike!
It's a great seasoned salt with overtones of curry & undertones of citrus. Made by Gayelord Hauser; available at most healthfood stores along with its sister seasoning Vegit, which is made with a special yeast base (great on salads, eggs, veggies, popcorn).
Cold steak sliced thin across the grain, coupla slices of quality cheese, toasted semolina or multigrain bread, a slather of good mayo (or maybe smoky/creamy horseradish sauce), a few shakes of Spike, perhaps some nice field greens. Nothing else necessary.
I like fourunder's simple broil idea. I can feel those tasty, crisp edges from the boiler.
I get really stupid with leftovers. Thoughts with flank and provolone are...
A quick reduction/braise with pineapples and sprouts.
Small pan. Put in some stock, wine, flank and sliced pineapples. Cook until tender, adding stock to keep up the liquid. Toss in sprouts, cook for a minute, then put cheese on top to melt.
Because I like eggs.
Brown onions, add flank, add worcestershire sauce. Form and melt cheese on top. Set aside. Fry egg, sunny side. Put on top. Layer with raw onion.
Slice the flank, egg and bread it with hot spices, fry. Shred greens (kale, cabbage, etc), quick saute with balsamic. Layer with cheese and mayo.
re: King of Northern Blvd
I'm really a less is more kind of guy.....I like a traditional open-faced broiled sliced steak sandwich on toast .....maybe some sauteed mushrooms and onions and Long Hot Peppers...garlic butter or drizzled with olive oil and fresh lemon juice. If I want it hero style....then some Crusty Italian Bread would be necessary.
Consider Fajita inspired if you need more zip