I usually use the little frozen baby asparagus spears. They're thin and not nearly as likely to be tough on the ends. I roast them for about 15 minutes in the oven, drizzled with olive oil, then cut them into bite-sized pieces and add them to my casserole mixture.
I'm a purist. I like Ritz crackers crushed and mixed with melted butter and paprika as my topping for my asparagus casserole. To me, that's the perfect topping for this casserole. I do like to make sure the casserole is adequately seasoned though, so I always add some cayenne pepper and garlic to the soup mixture.