- With the rack in the middle position, preheat the oven to 400°F (200°C). Line a baking sheet with a silicone mat or parchment paper.
- In the microwave oven, melt 2 tbsp of the butter.
- On the baking sheet, toss the squash with the melted butter and nutmeg. Season with salt and pepper. Bake for 30 minutes or until the squash is very tender, stirring halfway through cooking.
- Meanwhile, in a skillet over medium-high heat, brown the lamb in the remaining butter for about 2 minutes on each side for rare, spooning the butter over the meat as it cooks. Season with salt and pepper. Let rest on a plate for 5 minutes. On a work surface, cut the meat into 24 thin slices. Set aside.
- In a bowl, combine the currants, celery and lemon juice.
- With a fork, lightly crush each squash cube, making sure to keep them separate. Place one crushed squash cube on each cracker. Top with a slice of lamb and some of the currant mixture. Garnish with celery leaves, if desired, and sprinkle with fleur de sel.