Alsatian-Style Caramelized Onion & Gruyère Tart

Preheat your oven to 200°C. Roll out 250g of raw shortcrust pastry dough onto a baking sheet lined with parchment paper. Lightly poke holes all over the pastry with a fork, then brush the surface evenly with 15g of Dijon mustard, leaving a 2cm border around the edges. Place the pastry in the refrigerator to chill while preparing the onions.

Practice 'The Claw Grip' to slice the onions safely and evenly. Curl the fingertips of your guiding hand inward toward your palm, keeping your knuckles forward as a guide for the knife blade. Rest the flat of your chef's knife against your knuckles, and slowly slice 3 large yellow onions in half, then into thin, uniform half-moons, moving your hand back gradually.

Melt 30g of unsalted butter and 15ml of olive oil in a large skillet over medium heat. Add the sliced yellow onions, 1/2 tsp of salt, and 1/8 tsp of baking soda. Stir well to combine.

Cook the onions over medium heat, stirring frequently. The baking soda will accelerate the Maillard reaction, softening and turning the onions a rich, golden brown in about 15 to 20 minutes. In the last 5 minutes of cooking, stir in 1 minced garlic clove and 5g of fresh thyme leaves. Season with black pepper, then remove from heat to cool slightly.

In a medium bowl, whisk together 100g of crème fraîche and 1 large egg until smooth. Season with a pinch of salt and pepper, then stir in 50g of grated Gruyère cheese.

Spread the cooled caramelized onions evenly over the chilled pastry sheet, staying within the mustard-brushed area. Pour the crème fraîche and cheese mixture over the onions, spreading it gently. Fold the 2cm pastry edges slightly inward to create a rustic border, then sprinkle the remaining 50g of grated Gruyère cheese over the top.

Bake the tart at 200°C for 20 to 25 minutes, until the pastry border is golden brown and flaky, and the cheese topping is bubbly and deeply caramelized.

While the tart bakes, prepare the side salad. Whisk 15ml of balsamic vinegar de Modena, 5g of honey, 30ml of olive oil, and a pinch of salt and pepper in a large bowl. Add 100g of mixed salad greens and 5g of finely chopped fresh chives, tossing gently to coat.

Remove the tart from the oven and let it rest for 5 minutes. Slice into wedges and serve warm alongside the dressed mixed salad.