Is that 2 cups of sliced Brussels sprouts or 2 cups of whole brussels sprouts, sliced?
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When they are sliced, the pieces should fit in about 2 cups.
Since it says "2 cups Brussel sprouts, thinly sliced" instead of "2 cups thinly sliced Brussel sprouts", i would measure before slicing.
If you don't have a recipe, sauté the sliced sprouts in olive oil and butter with bacon and onion ... a great side dish.
I'd say run enough Brussel sprouts through a mandoline and measure out 2 cups of the sliced shreds
Oi vey! Is it really that critical? Use the measurement that fits how many Brussels you want to eat. At the end of the day the difference is probably only 1/2 a cup. Enjoy!