Another method that I sometimes use for flatfish shrimp or squid (basically works for anything that’s not too thick):
Prepare however you want:
remove the skin off the fish if you like
peel the shrimp or don’t
clean the squid (NOT optional) and slice into rings (recommended).
marinade for half an hour or so in olive oil, some pepper, a small chopped onion and the juice of at least one lemon.
put everything on a baking sheet or shallow oven-proof dish (make sure there’s plenty of moisture) and set under the grill in the oven for a few minutes until the squid turns opaque white/purple, the fish is done (see other postings) or the shrimp has curled up and has turned opaque (peeled) / pink (in the shell). Squid and shrimp shouldn’t take much more than 5 to 6 minutes, fish will probably take longer depending on size.
You will probably need to turn the shrimp and squid once to make sure it’s done on both sides. Same for thick fish.
This is my preferred method for making sure squid turns out tender and tasty instead of like rubber bands.
Serve with some fresh cilantro if you like.