1. Heat 1 1/2 teaspoons olive oil in a medium skillet over high heat. Add onion and cook, stirring, until golden, 2 to 3 minutes. Add mushrooms and cook, stirring, 2 to 3 minutes. Reduce heat to medium and add 1 tablespoon water; season with salt and pepper and cook until mushrooms are tender, about 5 minutes more.

2. Remove from heat and transfer to the bowl of a food processor; pulse 2 to 3 times to coarsely chop. Set chopped mushroom mixture aside.

3. Add remaining 1 1/2 teaspoons olive oil to skillet and heat over high heat. Add leek and season with salt and pepper; cook, stirring until leek is tender, 3 to 5 minutes. Add chopped mushroom mixture to skillet and stir to combine. Remove from heat and add vinegar; stir to combine. Transfer to a bowl and let cool to room temperature before using.