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Sam is a trusted home cook.
Heat up a cast iron skillet to med low. Turn on the broiler.
Put in the slice and cover so it warms the interior and crisps up the bottom crust. Remove lid and put under broiler to re-melt the cheese. For frozen, let it thaw or defrost in microwave and do the above to crisp up the crust.
Defrosted in microwave, followed sams directions and it was awesome!
Monita is a Recipe Tester for Food52
Or; let it thaw; pre-heat oven to 350 and then place pizza slice directly on oven rack (or with foil below) and warm up. Bottom may not be as crisp as cast iron method but a little less work
Just put the slice directly on the wire rack of a toaster oven, and heat until the top cheese melts a bit, a minute or so (more if the slice is refrigerator cold).
My secret ingredient for re-heating pizza (from the fridge or freezer) is water. Follow Sam's directions above, but first run the edge crust under a stream of water then dampen the bottom crust with a wet hand. Pour a drizzle of olive oil into the preheated skillet if you like. And instead of running under a broiler another option is to put a lid on the skillet to thoroughly warm the pizza. Cook until the bottom crust is crispy. I like pizza re-heated this way even better than pizza right out of the oven. I have just shared one of my greatest culinary secrets ;)
The cast iron method is great for droopy soggy bottom delivery pizza. Hey! we don't all make pizza from scratch every time.
A few herbs (dried or fresh) and some fresh onions or bell pepper and mushrooms really wakes up take out pizza.
I'll try the water method Sadassa mentioned..I have been known to toss in a tsp of water directly in the pan to steam for the blast at first..then broil.
And Shhhh...this even works well on the supercheapo Little Ceasar 5 dollar pizza. Yes, I've been broke before.
Wow! Reheating left over pizza !I've always enjoyed cold pizza in the middle of the night. Never have any left to reheat.
Just another option to the good ones already here -- let it thaw at least a bit. Sprinkle some cornmeal in a skillet on medium/medium low, and reheat the pizza on top of the cornmeal, giving it a good shake once in a while.
I always have my pizza stone in my oven I just preheat the oven and put the pizza on the stone!