Fresh tasting pasta sauce using canned crushed tomatoes as described on America's Test Kitchens. Use either Muir Glenn or Tuttoroso crushed tomatoes because they use a lower processing temperature in the canning process and therefore are fresher tasting than other brands. Recipe Notes: According to the chefs, grating the onions instead of chopping allows them to carmelize more quickly. The butter is used because saturated fats helps the onions stay sweet. Olive oil contains unsaturated fats that react with an enzyme in the onion that can result in a bitter taste. Using crushed tomatoes is preferable to other forms. The chefs apparently expended a great deal of effort in developing this simple recipe so it is suggested that it be followed closely in order to acheive a fresh tasting sauce from canned tomatoes.