Trim mushrooms--remove tough ends. Cut into a 1/4-inch dice. If you dice them too small, they'll turn pasty.
Put butter (or oil) in skillet and turn heat to medium. A minute later, add onion and garlic. Cook till softened, stirring occasionally, about 5 minutes. Add mushrooms. Raise heat to med-high, and cook until lightly browned, about 10 minutes.
Remove from heat and stir in lemon juice and sour cream. Season with salt and pepper. Garnish with dill.