Bring a large pot of salted water to a boil. Drop hearty greens into water and blanch for 4 minutes. Immediately plunge blanched greens into a bowl of ice-cold water to stop the cooking. Drain, squeeze dry, and set aside.
Heat oil and garlic in a large skillet over medium-low heat. Cook until the garlic is golden around the edges, 8 to 10 minutes. With a slotted spoon, remove garlic from skillet; set aside.
Raise heat to high. Add hearty greens and cook for an additional 5 minutes. Add spinach; season with salt and red pepper flakes to taste. Cook until spinach wilts, stirring frequently (about a minute). Return garlic to pan and stir. The greens can be served immediately or at room temperature.
Blanching does not actually cook the greens but softens their texture, while shocking them in cold water keeps their color strong and vibrant.