Combine yolks with oil and beer. Gradually stir this into flour (it should be thick enough to stick to the food) and beat until very smooth with a few swift strokes. Rest batter, covered, in refrigerator for 3 hours at least so it ferments a bit. Get oil to right temperature. Beat egg whites until stiff and fold into batter. Dip the onions or mushrooms in the batter and fry.