| Heat
the butter and olive oil in a deep, heavy stockpot over medium-high
heat. Add the onions and thyme sprigs and sauté the onions until
soft, about 5 minutes. Reduce the heat to medium-low and continue
to cook the onions, stirring occasionally, until the onions
begins to turn golden and caramelize, 1 to 1 ˝ hours. Add 2
cups of the beef stock and stir. Continue to cook the onions
for another 2 hours until they are a deep, dark color and most
of the stock is absorbed. Add additional stock if necessary.
Season to taste with black pepper and salt. Note: use caution
with salt. The beef stock may already contain enough salt for
your taste buds.
The
caramelized onions will keep, covered and refrigerated, for
up to two weeks.
In a
deep, heavy stockpot, over medium-high heat, add the beef
stock. Add the caramelized onion mixture and stir. Cook the
soup until it is hot and simmering.
Heat
the broiler. Butter and then toast the bread slices under
the broiler.
Ladle
a portion of soup into individual serving soup crocks. Place
a toasted bread slice on top of the soup and sprinkle some
of the grated Gruyere cheese on top of the bread.
Place
the soup crocks under the broiler until the cheese is melted
and bubbling hot. Serve immediately.
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