Decide your target: broccoli rabe bitterness (closest), mild green (broccolini), or “no-bite but garlicky” (spinach).
Use the ratio in the table: match 1:1 for greens with similar bitterness, or scale to 2:1 for spinach to replace lost bite/volume.
Match cooking time to leaf toughness: quick-sauté broccolini; simmer stems on kale/turnip greens briefly before adding leaves.
Finish the same way broccoli rabe is usually finished: lemon juice (or vinegar) + lots of garlic and a pinch of red pepper flakes.
Taste and adjust bitterness: if the substitute is too sharp, blanch briefly; if it’s too mild, increase lemon and garlic rather than salt.