Squeeze the juice of half the lemon into a bowl of water large enough to hold the artichokes. Trim the artichokes to the hearts, rubbing the cut surfaces with the lemon half to keep them from browning. Remove the choke, cut the artichokes into 1/4-inch thick slices, then toss the slices into the acidulated water.
In a 12 to 14-inch saute pan, heat the olive oil over medium-low heat and add the onion and garlic. Sweat the vegetables in the oil until they are soft and just golden brown. Add the fava beans and stir to mix well. Drain the artichoke slices then add to the pan with the favas. Add 1/2 cup water and increase the heat under the pan so that the liquid boils. Cook until the vegetables are tender, about 15 minutes. Season with salt and pepper and serve immediately.