Preheat oven to 350 degrees Fahrenheit. Lightly grease a 9x13 casserole dish and set aside.
In a salted pot of boiling water, cook pasta according to package directions for al dente pasta. Drain well, pour pasta into the prepared 9x13 pan, and set aside.
Meanwhile, in a saucepan over medium heat, add butter. Once hot and melted, add diced onions. Cook until onions begin to brown and turn translucent. Add garlic and flour, then cook for 1 additional minute.
Add chicken broth and milk, whisking continuously until incorporated and smooth. Bring to a simmer and cook until sauce has thickened slightly, about 3 to 5 minutes.
Remove from heat and add salt, Italian seasoning, red pepper flakes, and zucchini, stirring to combine. Pour mixture over noodles and gently toss to coat.
In a bowl, combine bread crumbs, melted butter, and salt. Sprinkle evenly over casserole.
Bake uncovered for about 15-18 minutes, or until casserole is hot and bubbling in the center. Optionally, broil for 2 minutes to brown crust on top. Do not walk away from your oven while broiling, your crust will brown quickly.
Top with fresh chopped chives (or your favorite fresh herb) just before serving.
