Gratin de brocolis

• 2 pounds (900 g) boiling potatoes, peeled
• 2½ to 3 pounds (1125–1350

Slice the potatoes into rough 1-inch (2½ cm) square pieces and cook them in a large quantity of boiling salted water for 15 to 18 minutes, until very tender when pierced with a knife. Drain thoroughly (reserve the cooking water).

While the potatoes cook, use a small, sharp knife to trim the stems of broccoli and peel away the tough, stringy skin. Halve or quarter the stems, depending on