Caramelize the onions and fennel: Heat olive oil in a large skillet with a tight-fitting lid over medium-high. Add the fennel and onion and season with salt and pepper. Cook, stirring occasionally until mixture begins to brown, about 5 minutes. Reduce heat to medium, cover, and cook 5-7 minutes. Uncover, add 1 Tbsp water and cook, stirring constantly, until golden brown and soft, about 2 minutes