Heat a large pot of water over high heat while you stem and wash the greens in two changes of water. Meanwhile, fill a bowl with ice water. When the water comes to a boil, add a generous tablespoon of salt, and the greens.
Blanch spinach for 30 seconds only, chard and beet greens for 1 minute, or until tender, and transfer to the ice water. Let sit for a few minutes, then drain, squeeze dry, and chop.
Heat 1 Tbsp. of the oil in a 10-inch non-stick skillet over medium heat and add the garlic.
COOK, stirring, until fragrant, 30 seconds to a minute, and stir in the greens.
COOK, stirring, for about 1 minute, until coated with oil.
SEASON to taste with salt and pepper and remove from the heat.
BEAT the eggs in a bowl, stir in 1/2 Tsp. salt, freshly ground pepper to taste, the milk, and the cooked greens.
CLEAN and dry your pan and return to the stove. Heat over medium-high heat and add the remaining olive oil. Hold your hand above it; it should feel hot. Drop a bit of egg into the pan and if it sizzles and cooks at once, the pan is ready.
POUR in the egg mixture, scraping every last bit out of the bowl with a rubber spatula. Swirl the pan to distribute the eggs and filling evenly over the surface. Shake the pan gently, tilting it slightly with one hand while lifting up the edges of the frittata with the spatula in your other hand, to let the eggs run underneath during the first few minutes of cooking.
TURN the heat down to low and cover the pan. Cook 8 to 10 minutes, shaking the pan every once in a while, until the frittata is just about set. Meanwhile, light the broiler.
PLACE under broiler for about 5 minutes to set the top.