Bring a large pot of salted water to a boil. Fill a large bowl with ice and cold water. Add leaves to boiling water; as soon as water returns to a boil, use tongs to remove leaves from water. Submerge leaves in ice water, then transfer to colander to drain; transfer leaves to a large bowl. Add stems to the same pot of boiling water; cook until almost tender, about 2 minutes. Drain stems and submerge in ice water, then drain again; add to bowl with leaves.

 

Combine oil, garlic and red pepper flakes in a large skillet; warm over low heat, stirring occasionally, until garlic begins to color, about 2 minutes. Add leaves and stems to pan, turning occasionally, until leaves and stems are warmed through and stems are tender, about 5 minutes. Season to taste with salt and pepper. Serve warm.