Melt butter in frying pan; add onion and cook until soft. Move onions to side and add flank steaks; fry on each side 1-2 minutes for medium rare, longer if desired. Remove steak and set aside.
To onions add salt and pepper, water and tea. Bring to a boil, and heat until juices have disappeared, stirring constantly, scraping up all browned bits from the bottom. Onions should be a rich caramel-brown colour. (The tea contributes to this colour as well as adding flavour.)
Slice steak thinly at an angle. Split multigrain rolls, creating a hinge. Place steak on roll; add a generous portion of onions, and tomato and lettuce (if desired), and top with grainy mustard. Makes 4 servings.