1. Cook egg noodles until al dente according to package directions. Drain, rinse with cold water, and drain again.
2. Melt butter in a medium saucepan over medium heat. Add leeks and cook, covered, 7 minutes or until tender but not browned. Whisk in flour; cook 1 minute. Pour in milk; bring mixture to a boil over medium-high heat, whisking frequently. Reduce heat to medium-low, and simmer 3 minutes or until sauce is smooth and thickened.
3. Remove from heat, and whisk in cheese and next 3 ingredients. Fold in tuna and egg noodles. Pour into a greased 3-quart baking dish.
4. Combine olive oil, garlic, and panko; sprinkle over casserole. Bake at 375° for 30 minutes or until golden brown and bubbly. Let stand 5 minutes before serving.