Mushrooms and onion rings.
Mini crushed roast potatoes.
New potatoes - boil for 10 mins (you can do this ahead) when the potatoes are cooked get a fork and partially crush them.
15 mins before you eat heat oil in the oven and add the potatoes, you can add honey, herbs, salt and pepper for a change.
Easy sauce for anything - melt cream cheese in a hot pan with a tablespoon of water.
You can add to this, so sliced and fried mushrooms, then add the cream cheese.
If you are frying the steak, remove it to rest, add a teaspoon of wholegrain mustard and cream, warm through and pour over your steak.
Salad goes well with steak.
I've just reread and noticed you are dieting so maybe a baked potato and salad - but make / buy a nice dressing.
You can also pour 1/2 glass of red wine into the pan you have used to cook the steak.
A mix of root veg mashed together.
Potato Rosti.
Chips but made with sweet potato, parsnips or carrots.