Preheat oven to 210C. Line an oven tray with baking paper.
Steam or microwave spinach until just softened. Squeeze out excess moisture; set aside.
Heat the oil in a frying pan; add the onion, cook until softened (3 minutes). Add remaining vegetables; cook until softened (5 minutes). Set aside.
Lightly brush one sheet of filo pastry with margarine, top with another sheet. Repeat layers with remaining sheets of pastry, alternating brushing with margarine.
Place the cooked vegetables, basil and cheese along one long side of pastry (5cm in from edge). Fold over the long sides of pastry, then short ends and roll up tightly.
Place strudel seamside down on baking tray; brush with remaining butter. Bake for 25 minutes (or until golden crisp).