- Preheat the oven to 375°F. Grease an 8-inch square baking dish with non- stick cooking spray or butter. Line with a piece of parchment paper, leaving an overhang on two sides.
- Combine the brick of dates with water in a small pot. Cover and bring to a boil, then reduce heat and simmer gently, covered, for 5 minutes or until the brick is soft enough to break apart. Stir well, then simmer, covered, another 5 minutes or until the dates are very soft. Stir well again. The pot should be fairly dry and the dates very pasty. If not, cook another 2 to 3 minutes. Remove from the heat and stir in the lemon zest and juice.
- Stir the flour, brown sugar and baking soda together in a large bowl. Stir in the oats and coconut. Using the large holes of a box grater, grate the butter right into the oat mixture. Stir until each flake of butter is well coated with oat mixture.
- Transfer about half of this mixture (about 3 1⁄2 cups/425 g) to the prepared dish. Press it firmly and evenly into the dish. Top with the date mixture, spreading it evenly. Sprinkle evenly with remaining oat mixture.
- Bake 45 to 50 minutes or until golden. Let cool completely on a rack, then gently pull upwards on the overhanging parchment to remove the square from the dish. Cut into 16 pieces.
Tip: You can make the crumble in a food processor instead of by hand. Whirl flour, brown sugar and baking soda until well combined. Add oats and coconut and pulse two or three times, until just combined. Cut butter into cubes, add to the food processor and pulse several times, or until mixture forms coarse crumbs.
Make ahead: Cool cooked squares completely, then wrap unsliced squares first in foil, then in plastic wrap. Freeze in a large resealable plastic bag for up to 1 month. Thaw, still wrapped, at room temperature before cutting.
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