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1
Peel onions and chop in half.
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2
Lay each half of the onion flat side down and slice onion in 1/4 slices.
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3
Separate layers of the slices so you end up with about 6 cups of half-moon shaped pieces from 3 medium size onions.
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4
Heat butter in a large frying pan until melted and add onions.
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5
Saute for 5-10 minutes until onions begin to soften.
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6
Add salt, pepper, vinegar, and Worcestershire sauce and place heat on medium.
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7
Cook onions for another 30 minutes or so, stirring often, until onions are very soft and cooked through.
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8
While onions are cooking, preheat oven to 375 degrees F and line 2 cookie sheets with parchment paper.
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9
While onions are cooking, slice baguette on the bias in slices a little less than 1/2 thick.
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10
Spread one side of each piece of baguette with a thin layer of mustard and place on the cookie sheets, mustard-side up, fitting as many onto each pan as you can.
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11
Once onions have caramelized, place some onions on top of each piece of baguette, enough to include onion in each bite.
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12
You can add more or less to taste, depending on how many onions you chopped.
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13
Sprinkle a tablespoon or two of grated cheese on top of the onions on each piece.
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14
Bake crostinis in the oven for 15-20 minutes or until cheese has melted and begun to bubble.
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15
You can also finish with a minute or two under the broiler, but keep a close eye on them to make sure they dont burn.
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16
Serve warm!
