Slice about two third of artichokes into small wedges; chop remainder into paste. Set aside. While pasta is cooking, saute onion and garlic in water until soft. Add oregano, salt and fresh-ground pepper. Reduce heat to low and stir in wine, lemon juice, spinach leaves (if desired) and reserved artichokes. Simmer, stirring, until spinach is wilted and all is heated through (1-3 minutes). Stir in ricotta. Drain pasta and add to sauce; remove from heat and add lemon zest just before serving.