How do I clean grease from crevices of nice baking sheets?

Try sprinkling them with baking soda and vinegar. Let it sit and scrub with a sponge or plastic scrub pad.

Here's a post that might help - https://www.webstaurantstore.com/blog/2404/how-to-clean-baking-sheets.html

Try a degreaser like GP66 or marble paste.

https://www.cleanipedia.com/gb/kitchen-cleaning/how-to-remove-burnt-on-grease-from-ovens-and-pans.html

https://www.youtube.com/watch?v=KaBihbVz3xM

I have used the ammonia method of removing burnt on grease from burner grates, it should work as well for cookie sheets.

Do this outside-place the cookie sheet in a sturdy bag (maybe contractor's grade) add 1/2 cup ammonia, seal it up and let it set overnight-at least 8 hours preferably in the sun. It is the fumes that do the work, not the actual ammonia liquid. With protective gloves on, remove the cookie sheet, rinse well with clear water.

I have always had good luck with oven cleaner. Spray on them cover with plastic wrap over nite. Then wash in hot soapy Dawn water.

Hi Theresa: What I use is a paste of vinegar and cream of tartar (the dry stuff). Make a paste of vinegar and cream of tartar (use just a bit of vinegar at first as the cream of tartar sort of melts) and put it on the gunk. Maybe start out with 3 tablespoons of cream of tartar and a 1/2 teaspoon of vinegar. Add more of whatever to make a paste. It should stick to a vertical surface if it's thick enough. Let it sit for about 15 or 20 seconds and then scrape off with a plastic putty knife, one of those plastic scrubbies or, for curly places, use a stiff brush or tooth brush. You may have to do it a couple of times if the gunk is thick, but usually not. There is no smell and it is environmentally safe. Good luck.