To get overwintered spinach or other hearty, dark leafy greens (i.e. kale, broccoli raab), farmers and home gardeners plant the greens in the fall. The shoots and leaves die off when the first frost arrives, but the roots survive by storing sugars. With the warmer days (relatively speaking) of March and April, stems and leaves appear, sweetened by the roots' sugars. (Conversely, mid-summer greens can be very bitter.)
For dinner last night, I sautéed overwintered broccoli raab and used it as a bed for two fried eggs and some cubes of cheddar cheese. The vegetable was ridiculously sweet. I do not cook overwintered spinach, though; its sweetness is completely lost by cooking and is much tastier when eaten raw.