Let the steaks rest and warm up on the cutting board for 15-30 minutes before cooking.
Pat steaks dry with a paper towel and season generously with salt and pepper on both sides.
Mushrooms and Onions:
Preheat a well seasoned cast iron skillet over medium heat and add butter. Once butter is mostly melted, add sliced onions and cook until onions start to brown.
Press in garlic and saute until fragrant. Add mushrooms to the pan and cook vegetables for a few minutes.
Pour in mushroom stock and season with salt, pepper, and thyme. Cook until mushrooms soften, take everything out of the skillet and set it aside.
Wipe the skillet down with a paper towel to get rid of any small pieces left behind.
Cooking Rib Eye Steaks:
Preheat the oven to 450°.
Preheat the skillet over medium-high heat and add a little bit of butter. Spread butter all over the pan and place the steaks in the hot skillet. Sear steaks for 30-45 seconds on each side.
Spread mushrooms and onions mixture over the steaks and place the skillet in the oven. Bake until the steaks reach your desired temperature. Depending on the steak thickness and temperature wanted, it could take between 5 and 15 minutes.
You can use a leave-in thermometer to take temperature of the steak but make sure to slide it through the side of the steak, towards the middle. (If leave in thermometer is not available, take the temperature with a regular meat thermometer but make sure to take the steak out of the oven and close the oven door while taking the temperature.)