In a large skillet, heat oil over medium heat. Add onion; cook and stir until softened, 4-5 minutes. Reduce heat to medium-low. Cook, stirring occasionally, until deep golden brown, 30-40 minutes. Remove from heat. Let cool slightly. Gently stir in goat cheese and tomatoes., Preheat air fryer to 375°. On a lightly floured surface, roll dough to 1/4-in. thickness. Cut with a floured 3-in. round biscuit cutter. Place 1 heaping teaspoon filling on half of each circle. Brush edges of pastry with water; fold circles in half. With a fork, press edges to seal., In batches, arrange empanadas in a single layer on greased tray in air-fryer basket; spritz with cooking spray. Cook until golden brown, 4-5 minutes. Turn; spritz with cooking spray. Cook until golden brown, 4-5 minutes longer.