Cook kale in a large pot of boiling salted water (1 Tbsp salt for 6 quart water) ntil just tender, 10 to 13 minutes, then drain.
Cook bacon in a 4-qt. heavy saucepan over medium heat until crisp. Transfer with a slotted spoon to a paper towel to drain. Discard all but 2 Tbsp fat from pan, then cook scallions until soft, about 3 minutes. Stir in kale and mustard and cook over medium-low heat, covered, stirring occasionally, until very tender, about 10 minutes. Serve topped with bacon.