To cook leeks heat the olive oil in medium skillet over meduim heat. Add the leeks and saute stirring, until limp and lightly browned about 15 min. Spread the leeks evenly in the bottom of an oiled 2 qt. shollow gratin dish. Let cool. To cook the potatoes in a medium saucepan, cover the potato slices with well salted water and bring to a boil. Reduce the heat to a gentle boil and cook for 5 min. or until the potatoes are just barely tender. Drain and rinse under cold water until cool. Pat dry. Toss the potatoes with the salt, olive oil and rosemary. To assemble the gratin heat the oven to 375 . Sprinkle overlapping tomato slices across the width of the dish. Prop the tomatoes against the dish at a 60 degree angle. Cover the row of tomatoes with a generous sprinkling of Gruyere. Next, arrange a row of potato slices over the tomatoes. Sprinkle atain with Gruyere. Repeat with alternating rows of tomatoes and potatoes, sprinkling each with cheese, until the gratin is full. Sprinkle about 1/2 tsp. rosemary over all and season with pepper. Drizzle with the olive oil.Mix any remaining Gruyere with the breadcrumb mixture and spread this over top. Place in over and cook until the juices have bubbled for a while and reduced considerably, approx. 60 to 65 min. Let cool for at least 15 min. before serving.