Preheat oven to 350°F. Cook pasta according to package directions to al dente. Drain and set aside.
In large saucepan, melt butter over medium heat. Add flour and cook 3-4 minutes; whisking frequently. Add mustard and milk, whisking to incorporate. Slowly add cheese, salt and pepper, whisking to combine. Reduce heat to low and cook 3 minutes.
In large skillet, fry bacon until crisp. Remove bacon to paper towel to drain; reserving 1 tablespoon bacon drippings. Crumble bacon once cool.
Sauté leeks in reserved bacon drippings until translucent. Add bacon, leeks and tomatoes to cheese mixture. Stir gently to combine.
Pour pasta into buttered 2-quart baking dish; pour cheese sauce over top. Gently stir to combine. Bake 35 minutes or until top is golden and bubbly.