Heat oil in a large (12-inch) skillet over medium-high heat. Add onions, tomatoes, fennel seeds, and 1/2 teaspoon of salt and a few grinds of pepper; cook, stirring frequently, until onions are tender and tomatoes start to break down, about 5 minutes. Add garlic; continue to cook until fragrant, a minute or so longer. Stir in tomato paste; cook to intensify flavors, another minute or two. Stir in white wine and cook until almost evaporated, another minute or so. Stir in chickpeas and spinach, followed by the rice and oregano; cook to heat through. Serve immediately with optional feta and a sprinkle of oregano.