, stems trimmed and chopped, about 16 cups packed
thinly sliced red
Bring a large pot filled two-thirds with water to a boil. Add rapini to water and boil, uncovered, for 1 min (do in two batches, if needed). Drain well. Melt butter in a large frying pan over medium-high. Add onion and garlic. Cook until onion is soft, about 3 min. Add rapini and salt, stirring often, until rapini is coated with butter mixture, 1 to 2 min. Transfer to a platter.