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It is not necessary to use a non-reactive pot. When the water is boiling, add the broccoli rabe stems. Cook for 2 minutes. Cook both stems and leaves for 1 to 2 minutes more.  Take the broccoli rabe out and pat it dry with a paper towel or clean kitchen towel.
Add water to a pot and boil. Add the leaves after those 2 minutes are up. Strain the broccoli rabe from the boiling water and immediately place the stems and leaves into a bowl of ice water. Leave the broccoli rabe in the ice water for several minutes.