Snap off the dark outer leaves of the artichoke and peel the stem. With a sharp knife, cut off the top to within about 1” of the base. Remove any remaining dark leaves, so that only pale green leaves remain. Rub the peeled artichoke with a cut lemon to prevent discoloration. Cut the artichoke in half and scrape out the choke with a paring knife or melon baller. Place prepared artichokes in a bowl of lemon ice water while trimming remaining artichokes.
Heat the olive oil in a sauté pan, add onions and sauté until softened. Add garlic, fennel seeds, tomatoes, salt and pepper and bring to a simmer. Drain artichokes and add to the pan, arranging in a single layer; turn to coat each with the sauce. Cover, reduce heat to low and simmer 15-20 min, until tender but not mushy, turning two or three times to ensure they cook evenly. Add a little water, if needed. Adjust seasoning to taste, then arrange on a platter. Serve warm or at room temperature.