2 Lemons, 1 Orange, 1 Apple — The Only Bars I Bake Anymore

INGREDIENTS
Units: 1 cup = 240 ml, 1 tbsp = 15 ml, 1 tsp = 5 ml.


DOUGH
All-purpose flour - 2 1/3 cups (280 g)
Butter, unsalted, cold - 1 1/3 sticks (5.3 oz / 150 g)
Sugar - 1/4 cup (50 g)
Full-fat sour cream or whole-milk Greek yogurt - 3 tbsp (50 g)
Baking powder - 1/2 tsp (2 g)
Vanilla extract - 1 tsp
A pinch of salt

FILLING
Lemons - 2
Orange - 1
Apple - 1
Sugar - 3/4 cup (150 g)
Cornstarch - 1/4 cup (30 g)

Pan: 8x8 inches (20x20 cm). Yield: 12-16 bars.

INSTRUCTIONS
1. Stir the sugar with the sour cream (or Greek yogurt) and vanilla, then chill it - it goes into the flour cold.
2. Sift the flour with the baking powder so there are no lumps and that tiny amount spreads evenly. Add the salt and mix; salt makes everything taste more buttery.
3. Cube the cold butter small, toss it into the flour and rub it in with your fingertips until it looks like coarse crumbs. Work fast - softened butter just smears into the flour.
4. Add the chilled sour cream mixture, stir with a spatula, then switch to your hands and stop the moment it forms a soft ball - overworking makes the crust tough, not crumbly.
5. Split the dough in two, one piece slightly smaller (about 60/40), wrap the smaller one in plastic and freeze it.
6. Line the pan with parchment, overhanging two sides so the bars lift out. Press the larger piece over the bottom, smooth it evenly, push it slightly up the sides and chill it. An 8x8 square pan is ideal; a round one that size works too.
7. Zest 2 lemons, yellow layer only - the white pith is bitter. Cut them into wedges, cut away the skin and pith, chop the flesh small and pick out every seed: one blended seed turns the whole filling bitter.
8. Do the same with the orange - peel, pith and the membranes between segments - and add it to the lemons.
9. Peel and chop the apple small. It mellows the sharp citrus and adds pectin, which sets the filling. Any crisp, firm apple works (Granny Smith, Honeycrisp).
10. Add the zest you saved and blend until mostly smooth; a few little bits are fine.
11. Stir in the sugar and cornstarch, leaving no dry spots before it hits the heat - cornstarch added to a hot mixture lumps instantly.
12. Move it to a heavy-bottomed pan so it doesn't scorch. On medium heat, cook to the first bubbles, then stir for about 10 minutes, scraping the bottom and corners, until it thickens and stops looking watery - a spatula track should hold for 2-3 seconds. Cook it much longer and the starch breaks down and thins again. Taste, and add up to 3 tbsp sugar if too tart.
13. Scrape it into a clean bowl so it stops cooking, stirring now and then as it cools. It must be at room temperature before it meets the cold crust, or it melts the butter in the base.
14. Spoon the filling onto the chilled crust and smooth it corner to corner, leaving 1/2 inch (1 cm) clear at the edges - it spreads as it heats.
15. Grate the frozen dough over the top on the large holes and spread the shreds evenly, but don't press them down - the air between them makes the topping crisp. If it starts smearing, freeze it 10 more minutes.
16. Bake on the middle rack of a preheated 390F (200C) oven for 30-35 minutes, until evenly golden with no pale spots.
17. Cool all the way in the pan, at least 3 hours - the filling sets as it sits.
18. Dust lightly with powdered sugar if you like, then lift the bars out by the parchment and slice into neat squares, wiping the knife between cuts.

NOTES AND TIPS
Cold dough is everything: it only grates if it is rock hard. Freeze it at least 2 hours if you can, ideally overnight (wrapped, it keeps a month), and hold it through plastic wrap while grating.
Butter: European-style, 82-85% fat (Kerrygold, Plugra) gives the crumbliest crust. Standard 80% works, but freeze the dough 30 minutes longer.
Sour cream must be full-fat - light or fat-free adds water and makes the crust tough.
Weigh the flour (280 g) or spoon it into the cup and level it off - scooping packs in 20-25% extra and dries the bars out.
US lemons are usually waxed: buy unwaxed or scrub them with hot water first. Thick-skinned? Pour boiling water over the zest for 30-60 seconds and drain - less bitterness, same aroma.
Home ovens often run 25F (15C) off - an oven thermometer is the cheapest fix.
Storage: 4 days sealed at room temperature, 6 days in the fridge (sit out 30 minutes first), or 2 months frozen, layered with parchment.