Why did my tenderloin end up undercooked inside?

So last Thanksgiving, I tried (for the first time in a while actually) to cook a tenderloin and every single time I tried (I split the entire loin I bought into 5 4 oz-ish pieces) I kept charring or burning the outsides while the insides stayed rare/raw.
What I did before cooking it was I marinated the pieces in Ponzu a full day before taking it and putting on the pan. I used olive oil on a non-stick pan (?). I think I kept the heat at around medium-high and generally let it cook without moving the steak around too much.
I let it cook on each side for about a minute and a half just to be safe. Then when I finally cut steaks into slices, it was still mostly undercooked on the inside while being thoroughly cooked and charred on the outside.
I still served the steaks after cooking it again on the pan and giving it a squeeze of lemon over it...
Best Answer
I am going to guess that your 4 oz tenderloin was probably about an inch thick. Not moving that, in a hot pan, for only a minute and a half left it under cooked in the center and over cooked on the outside. The pan was too hot and the cook time was too short. Given your limited experience cooking tenderloin, I would suggest a couple of adjustments. First, get a thermometer to accurately measure the temperature of your steak. Identify the doneness you prefer and look up the associated temperature. If you are cooking in a pan, on the stove top, then turn the heat down to medium at the most...add some fat to pan. Cook your steak, flipping every 30 seconds for about 5 minutes. Check the temperature. Continue flipping regularly until you are 5 - 10 degrees below your target temperature. Remove from the heat, tent with foil, and allow to rest 5 - 10 minutes (carry over heat should get you the rest of the way to your target). With experience, you will gain a feel for the doneness and can eliminate the thermometer. Time is not an accurate measure, as there are too many intervening variables. So, get a feel for temperature instead; not only measured temperature, but actually touch the cooked meat to see what it feels like. Over time, with experience, this can guide you quite well.
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How do you fix a undercooked beef tenderloin?
Place the meat in an oiled roasting pan or Dutch oven; drizzle it with some stock, sauce, or water; cover it with aluminum foil; and bake the whole thing in a 400\xb0 F oven until cooked.Can you undercook beef tenderloin?
Beef tenderloin needs care when cooking. Undercook it, and it might be too cool in the center and too rare for some guests. Overcook it and, well, everybody will probably still eat it, but that's not the point. Here's a few easy steps for tenderloin success.Can you eat beef tenderloin rare?
Tenderloin is lean and one of the most tender cuts around, but the lack of fat means that overcooking it will result in dry, tough meat. Follow this tip: Tenderloin is best served rare or medium-rare, so use a thermometer to make sure it doesn't get cooked past 140\xb0F in the center.Carla Makes Roasted Beef Tenderloin | Bon Appétit
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Answer 2
When you marinate meat in something like Ponzu, which has a lot of sugar, it's important to dry the meat thoroughly before pan-frying or using another conductive heating technique. You were probably burning the sugars in the sauce that adhered to the outside of the meat.
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