We wanted a classic, steak dinner option which will feel celebratory any cozy night of the week or work well if you want to enjoy it on New Year's Eve. Smarts: Horseradish has a strong flavor that some people love to pile on and others enjoy in smaller portions. Taste and adjust the sauce as you go to make sure you find the right balance for your tastes.
Place potatoes in a saucepan and cover with cold water. Season water generously with some salt. Cover and bring to a boil.
While potatoes are coming to a simmer, spread green beans out on a sheet pan. Toss with olive oil and thyme leaves. Season with some salt. Roast in the oven, shaking the pan halfway through cooking, until the green beans are tender, ~15 minutes.
Return to the potatoes and remove the lid. Simmer, uncovered, until the potatoes can be easily pierced with a fork, ~15 minutes.
Meanwhile, heat a large skillet with cooking oil over medium heat. Sear steak on both sides until it reaches your desired level of doneness - Medium-rare: 130F / 54C degrees (~3 minutes per side); Medium: 140F / 60C degrees (~4 minutes per side); Well-done: 160F / 71C degrees (~6 minutes per side).
Remove steak from heat and transfer to a cutting board. Cover steak and let it rest for 10 minutes.
Drain the potatoes and let sit in a colander for ~5 minutes to allow steam to escape (this creates creamier potatoes vs. liquidy potatoes).
Use a masher to mash potatoes with sour cream (portion for potatoes), butter, and some salt.
Slice steak and serve it with horseradish sauce over top. Enjoy with mashed potatoes and green beans on the side.