2) In a large bowl, toss the vegetables with the cream, stock, tarragon, salt, pepper, garlic and Gorgonzola. Put into the gratin dish and cover loosely with heavy duty foil. Bake for 1 hour, until the vegetables are fork tender.
3) Once tender, remove the foil, leave the dish on the middle rack and turn the grill on high. Grill the gratin until the top is golden, 3 to 4 minutes. Allow to cool for 10 minutes before serving.
4) The gratin can be made 1 day ahead and reheated at 160C/Gas 3, covered with foil, for approximately 20 minutes.