Preheat the oven to 375 degrees F. Whisk the eggs, milk, 3/4 teaspoon salt and a few grinds of pepper in a large bowl until combined; stir in the cheese and set aside.
Heat the olive oil in a 10-inch ovenproof nonstick skillet over medium-high heat. Add the greens, garlic, rosemary and red pepper flakes and cook, stirring occasionally, until the greens are wilted, about 5 minutes. Season with salt and pepper.
Reduce the heat to medium. Pour the egg mixture into the skillet and stir once or twice to combine; cook, without stirring, until the edges start setting, about 1 minute. Transfer the skillet to the oven and bake until the frittata is set and lightly browned, 25 to 30 minutes. Let cool 5 minutes in the skillet, then invert or slide onto a cutting board. Serve warm or at room temperature.