Trim the tops and bottoms from the fennel bulbs, reserving about 1/4 cup of the fronds. Halve the bulbs; cut out and discard the tough cores. Cut into 1-inch thick wedges.
Heat the oil in a large skillet with a tight-fitting lid over medium-high heat until very hot but not smoking. Add the fennel and cook, stirring once, until the fennel is browned, about 7 minutes.
Add 1/2 cup water to the skillet, scraping up any browned bits from the bottom. Sprinkle with 1/2 teaspoon salt and several grinds of pepper. Lower the heat to medium, cover the skillet, and cook until the fennel is very tender, adding more liquid if needed, 10 to 12 minutes.
Meanwhile, zest half the lemon and juice the whole lemon; chop the reserved fennel fronds.
Remove the skillet from the heat and uncover. Stir in the lemon juice and zest. Scatter the chopped fronds over the top.
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