Step 1

Cut lemons lengthwise into eighths; remove seeds. Thinly slice crosswise into pieces (you should have about 1 1/2 cups).

Step 2

Heat oil in a large skillet over medium-high until shimmering. Add fennel, onion, fennel seeds, and a large pinch of salt. Reduce heat to medium; cook, stirring occasionally, until mixture is softened and just beginning to turn golden, 10 to 12 minutes. Add lemons, honey, and 1/3 cup water. Increase heat to high and bring to a boil; cook until reduced to a jammy consistency and liquid is almost completely evaporated, 4 to 5 minutes more. Transfer to a bowl; let cool completely. Refrigerate in an airtight container up to 5 days.