Best-Ever Mushroom Sauce

I'll be using this in the (upcoming) scalloped potatoes recipe from the same blog, but also think it would be lovely on steak, or mixed into pasta.

Butter
Onion, sliced
Mushrooms, sliced
Lemon Juice
Yogurt (approx 1 cup)
Sea Salt
Pepper

Melt butter, then add onion and saute until cooked through. Add mushrooms and saute, covered, until cooked about halfway (still slightly plump), then add yogurt, lemon juice and salt. Bring to simmer and let cook, covered, about 30 minutes, or until thickened. Add pepper at end.