This marmalade accompanies chef Ben Ford's recipe for Polenta Fries.
Bring a large pot of water to a boil over high heat. Prepare an ice-water bath; set aside. Using a small sharp knife cut an "X" in the core of each tomato. Place tomatoes in boiling water and cook until skins begin to split, 30 to 60 seconds. Drain and immediately transfer to ice-water bath. Peel tomatoes and coarsely chop; set aside.
Heat oil in a medium skillet over medium-high heat. Add onions and cook, stirring, until lightly browned, 2 to 3 minutes. Reduce heat to low and continue cooking until onions are softened, about 5 minutes more. Add chopped tomatoes, lemon zest, lemon juice, vinegar, brown sugar, and 2 tablespoons water to skillet. Cook, stirring occasionally, until thickened, 2 to 3 minutes. Remove from heat and transfer to a medium bowl. Refrigerate until chilled.