Preheat oven to 325 degrees. Wash greens by placing them in a pan of cool water, swishing them around, then lifting them from the water. If any dirt remains in the water, start with a new pan of water and repeat. Strip the leaves from the stems of the greens. Chop the stems into 1/4-inch pieces. Chop the leaves of the greens into bite-sized pieces.
In large saucepan, boil one quart water. Add stems, garlic, and onion. Stir, cover, and boil until tender, about 5-7 minutes. Add leaves to the pot. Stir and cover. Cook, stirring often, until greens are tender, about 3-7 minutes. Drain, pressing with fork to remove more water.
In a large bowl, beat eggs. Stir in milk, salt, and cheese. Add greens and stir (it will be thick). Pour into greased 9 x 13 inch casserole pan. Bake, uncovered, 35-40 minutes or until center is set. Serve warm or chilled. Can assemble the night before, and bake the next day. This recipe is wonderful for "greens novices", those who have never tried them! It's a crustless quiche that is very delicious!