Preheat oven to 180 C / Gas 4. Spray a 20x30cm or similar sized baking dish with cooking oil.
Spray a large frying pan with oil and heat over medium-high heat. Saute onion and garlic for 3 minutes or until onion is tender-crisp. Stir in stock and rosemary; bring to the boil. Stir in artichoke hearts and spinach; reduce heat, cover and simmer 5 minutes. Stir in pasta sauce.
Spread 1/4 of the artichoke mixture in the bottom of the prepared baking dish; top with 3 lasagne sheets. Sprinkle over 1/4 of the mozzarella. Repeat layers 2 more times, ending with artichoke mixture and mozzarella cheese. Sprinkle crumbled feta on top.
Bake, covered, for 40 minutes. Uncover and bake 15 minutes more or until hot and bubbly. Let stand 10 minutes before cutting.