For me, the best steaks are simple. Buy the best quality meat you can afford (my preference is filet mignon or ribeye), give it some salt and pepper, and then cook it. The two methods I use most for cooking the steak are either with a gas grill on high heat, 2-4 minutes per side depending on thickness, or I will sear 2 minutes per side in a cast iron skillet, then put in a 450F oven for another 4-6 minutes.
As for the mushrooms, there are others who can give you very good ideas. The other day I sauteed some in olive oil with salt, pepper, and thyme and they were delicious. (you can tell I am a rookie, my recipes are quite basic, but with quality ingredients they will taste great)