Preheat oven to 230°C. Line a baking tray with non-stick baking paper. Melt butter in a saucepan over medium-high heat. Add swede, parsnip, sweet potato and carrot. Cut open bouquet garni sachet and sprinkle over vegetables. Add stock, bring to the boil over a high heat. Cook, uncovered, stirring occasionally, for 15 minutes or until vegetables are tender. Drain.
Meanwhile, cut a 20cm-diameter disc from each pastry sheet. Cut each disc into quarters. Place on lined tray. Bake in preheated oven for 8-10 minutes or until puffed and golden.
Place a pastry piece on each plate. Spoon over some of the vegetable mixture, top with 2 teaspoons of horseradish cream and another pastry triangle. Sprinkle with rosemary to garnish. Serve with green salad.