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Preheat oven to 350 degrees F.
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Peel onions and slice them in half length wise. Place onions in a baking dish with cut sides facing up and drizzle them with olive oil. Season to taste with salt and pepper. Cover with foil and bake for an hour and a half.
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Remove foil from onions and bake for an additional 15 minutes or until onions begin to brown.
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Mix half and half, thyme, mustard and cheeses in a bowl. Spoon mixture evenly on top of onions. Bake for 15-20 minutes until onions are golden.
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When serving, be sure to spoon some extra cheese gravy onto each onion half.