I have never cooked the shells, but I recently saw an item on Frugal Life about cooking the shells. Can’t find the tip now. The Feed the Birds book by Helen & Dick Witty says to rinse out the shells of freshly broken eggs, scrape out remanants from the shells of soft-boiled eggs and rinse them; peelings of hard-boiled eggs are ok as is. Hope you enjoy making suet as much as I do. What is your recipe?