Preheat the oven to 350 degrees Fahrenheit.
In a large pot (I used a 6 quart), boil the water for the pasta. Meanwhile, gather and measure the rest of your ingredients.
When the water boils, add the pasta to the pot and following the cooking instructions on the box.
After the spaghetti has finished cooking, drain it and rinse briefly with cold water to prevent it from sticking and continuing to cook. Set aside.
In the same large pot, combine the half and half, broth, water and butter. Cook until the butter is completely melted. Add the flour and Italian Seasoning and bring the mixture to a boil, stirring constantly. Boil for 1 minute while continuing to stir. Taste the sauce and add salt, if needed.
To the cream sauce, add the 3 cups of vegetables, olives and the spaghetti. Stir to combine.
Carefully pour the tetrazzini from the pot into an ungreased 9 x 13" (3 quart) baking dish. Sprinkle the cheddar cheese evenly across the top of the casserole.
Bake at 350 for 30 minutes, or until the sauce is bubbling and the cheese melted.
Remove the vegetable tetrazzini from the oven. Allow to cool, then serve and enjoy.