How to keep shallots from overpowering vinegarettes
I always rinsed onions and shallots, to remove the compounds released when slicing through the cell walls. Since reading the following article in Cook's Illustrated March 2011 edition, I prefer its baking soda method.


  Toning Down Raw Onion’s Bite
  
  We’ve often heard the claim that soaking sliced or chopped raw onions in liquid can mellow their harsh taste by drawing out the pungent sulfur compounds known as thiosulfinates that are produced when the onion is cut. 
  
  But what kind of liquid and how long of a soak? We tested three of the most commonly recommended liquids—water, milk, and vinegar—by soaking the cut onions in each for 5 and 15 minutes. We found that 15 minutes was necessary for any of the treatments to be effective. 
  
  The vinegar soak did rid the onions of much of their burn, but it was replaced by an equally strong sour taste, even after thorough rinsing. Milk was also very effective at removing the sulfur compounds, but it left the onions tasting washed-out. 
  
  The best method—better than even plain water—was our own: a baking-soda solution (1 tablespoon per cup of water). Unlike the other methods, which merely do their best to leach away the offending sulfur compounds, the alkaline baking soda neutralizes sulfenic acid, the immediate precursor to the harsh-tasting thiosulfinates, and prevents them from forming in the first place. 
  
  Just be sure to rinse the onions thoroughly before using to remove any soapy baking-soda taste.