Adapted from Mollie Katzen's original recipe, Pasta with Greens and Feta, published in "Still Life With Menu." Can use any combination of bitter greens; in her notes the author states that she prefers half escarole and half spinach. When I made it I used two kinds of kale and also cooked the onions twice as long as written to really carmelize them and produce a richer flavor. The original recipe states that you can use up to a pound of pasta and 12 ounces of feta. I could see easily doubling this recipe, or tweaking amounts based on your tastes and the availability of ingredients. The author also notes that you can make the sauce and refrigerate it ahead of time, then reheat it while you're boiling the pasta. A delicious way to eat your greens, an excellent spring dish.