Soak dandelion greens in a large bowl of cold water with 1 teaspoon salt for 10 minutes. Drain.
Bring a large pot of water to a boil with 1 teaspoon salt. Cook greens until tender, 3 to 4 minutes. Drain and rinse with cold water until chilled.
Heat olive oil and butter in a large skillet over medium heat; cook and stir onion and red pepper flakes until onion is tender, about 5 minutes. Stir in garlic until garlic is fragrant, about 30 seconds more. Increase heat to medium-high and add dandelion greens. Continue to cook and stir until liquid is evaporated, 3 to 4 minutes. Season with salt and black pepper.
