Root Vegetable Strudel

Name: Root Vegetable Strudel
Ingredients: 4 cups (1-inch pieces) Delicata squash; 1 large celery root, peeled and cut into 1-inch pieces; 4 shallots; 4 portobello mushrooms; 2 tablespoons olive oil; -- Coarse salt and freshly ground pepper; 1/4 cup chopped, toasted walnuts; -- Creme fraiche (optional); 2 store-bought puff pastry sheets (1 pound total)