Are you talking about a filet, like filet mignon (beef tenderloin) or like a filet of fish.
For tenderloins, here's what I do (very unhealthy and very tasty).
1. Coat filets generously with S&P, and leave out of fridge for 30 minutes or more to get to room temperature (you're going to eat filet RARE and it's better if the rare parts aren't cold)
2. Get a pan wicked hot (preferably NOT nonstick), like a stainless saute pan or a cast iron fry pan.
3. Put in a bit of oil and butter, but not a ton, and let it get hot.
4. Put the filets in, just a couple minutes per side for something rare. Leave 'em in longer for medium. You should have a lot of smoke right now, but keep that pan hot.
Now, we get just slightly fancy, but this is a great technique...
5. Turn the heat to medium and DEGLAZE the pan by pouring 1/4 to 1/2 cup of beef stock (I use chicken stock which is easier to find in the grocery store) into the pan and stirring up all the browned bits still stuck to the pan (non-stick pans = less browned bits = less flavor).
6. Put in garlic and shallots (just a TBSP or so of each. maybe more shallots than that) and REDUCE the stock (boil it down) by about half. Keep stirring.
7. Now, over low heat or no heat, stir in heavy cream. Strain out garlic and shallots and pour sauce over meat.
You just made a version of Steak Au Poivre and your house smells like heaven right about now.
That might sound like a little work (deglaze? reduce? stock? cream? shallots?) but it's a great technique and you can do it with chicken, pork chops, lamb chops, and about 5 different cuts of steak. It's just a basic brown pan sauce. Takes about 5 minutes after your steaks are cooked.
The nice thing is, its very easy to individualize. In step 5, you can also add a little red wine (my choice), brandy or cognac, and light the brandy on fire.
In step 6 you can add any combination of garlic/shallots/onion/rosemary/sage/thyme/bay leaf or none of it.
In 7, you can add cream, butter, neither, both.
I prefer a fattier cut of meat, like a strip steak, for this dish.
They sell stock in the grocery store. You can freeze what you don't use. For heavy cream use "table cream", "heavy cream" or "whipping cream". Whipping cream and heavy cream are basically the same thing.
Chop up the garlic and shallots before you start the steak.
Serve with baked potatoes, mashed potatoes, roasted potatoes. even french fries.