Melt 2 tablespoons butter in a large skillet over medium-high heat. Add peppers and celery; sauté 8 to 10 minutes or until crisp-tender. Sprinkle with salt. Set vegetables aside.
Melt 1/2 cup butter in a large saucepan over low heat. Add flour, whisking until smooth; cook 1 minute. Gradually add whipping cream and half-and-half; cook over medium heat, stirring constantly, until thickened. Whisk in celery seeds and white pepper. Remove from heat; add blue cheese, whisking until cheese melts.
Whisk eggs in a medium bowl until lightly beaten. Gradually whisk about one-fourth of hot white sauce into eggs, whisking constantly; add to remaining white sauce, stirring constantly. Whisk in celery leaves.
Cook pasta according to package directions; drain and return to pan. Stir in vegetables and white sauce. Pour into a lightly greased 13" x 9" baking dish. Sprinkle with Parmesan.
Bake, uncovered, at 400° for 30 minutes or until bubbly and lightly browned. Let stand 5 minutes before serving.