In a large skillet, saute fennel in oil until softened. Reduce heat to medium-low; cook, uncovered, until deep golden brown, stirring occasionally, 40 minutes. Stir in the thyme, vinegar, salt and pepper. Preheat oven to 400°., Unfold each puff pastry sheet onto an ungreased baking sheet. Using a knife, score 1 in. from the edges of each pastry. Spread fennel mixture to within 1/2 in. of edges. , Bake until golden brown, 12-15 minutes. Cut each tart into 12 pieces. If desired, top with reserved fennel fronds.