According to the definition:

cas·se·role: kasuh-rohl
noun, verb, -roled, -rol·ing.
1. a baking dish of glass, pottery, etc., usually with a cover.
2. any food, usually a mixture, cooked in such a dish.
3. a small dish with a handle, used in chemical laboratories.
–verb (used with object)
4. to bake or cook (food) in a casserole.

the sentence “I casserole a casserole in this casserole.” would be valid. But trying saying it ten times fast…