Bend back outer petals of each artichoke until they snap off easily near their base. Edible portion of petals should remain on the artichoke bottom. Continue to snap off and discard thick petals until central core of pale green petals is reached. Trim brown end of stem and cut off top 2-inches of artichokes. Pare outer dark green surface layer from artichoke stems. Cut out center petals and fuzzy centers. Slice artichoke bottoms about 1/4-inch thick. Wash well in a large bowl of water rinse and repeat the washing a couple of times. Drain and set aside. In a large bowl, toss the artichokes with lemon juice to prevent discolouration. Meanwhile, in a large pan, sauté onions in olive oil for 5 to 8 minutes or until tender. Remove onions and place them in a layer at the bottom of the oven-proof baking dish. Sprinkle with oregano. Arrange tomato slices and artichokes over the onions. Cover dish with lid or foil. Bake at 375°F for about 30 minutes. Remove casserole dish from oven and sprinkle the grated cheese on top and place the dish back in the oven covered for about 6 minutes longer. Remove from the oven and serve as a side dish with any meat or fish. Makes 6 servings.
