Walla Walla Onion and Gruyère Tart

Walla Walla Sweet onions — grown in the volcanic soil of southeastern Washington's Walla Walla Valley since French-Canadian trapper Peter Pieri planted Italian Cipolla di Tropea seeds there in the 1900s — are so mild they can be eaten like an apple. When slow-caramelized, they develop a jammy, wine-dark sweetness that anchors this elegant open-faced tart, a staple at Pacific Northwest farmstead markets and dinner tables through summer and early fall.

Serves: 6

Ingredients

For the crust:

• 185g (1.5 cups) all-purpose flour
• 1/2 tsp (2g) fine salt
• 113g (1/2 cup) cold unsalted butter, cubed
• 3-4 tbsp (45-60ml) ice water

For the filling:

• 1.1 kg (2.5 lbs) Walla Walla Sweet onions (3-4 large), thinly sliced
• 3 tbsp (45g) unsalted butter
• 1 tbsp (15ml) olive oil
• 1/2 tsp (2g) fine salt
• 1 tsp (4g) sugar
• 2 tsp (10ml) fresh thyme leaves
• 60ml (1/4 cup) dry white wine or dry vermouth
• 3 large eggs
• 180ml (3/4 cup) heavy cream
• 100g (3.5 oz) Gruyère, grated
• 30g (1 oz) Parmesan, grated
• White pepper and nutmeg

Instructions

1. Make the crust: pulse flour and salt together. Add cold butter and pulse until pea-sized. Add ice water one tbsp at a time until dough just holds together. Flatten into a disc, wrap, and refrigerate 1 hour.
2. Roll dough on a floured surface into a 32cm (13 in) circle. Fit into a 23cm (9 in) tart pan with removable base. Trim and chill 30 minutes. Blind-bake at 190°C (375°F) with weights for 15 minutes, remove weights, bake 10 more minutes until pale gold. Cool slightly.
3. Caramelize the onions: heat butter and oil in a wide heavy pan over medium heat. Add onions, salt, and sugar. Cook, stirring every 5-7 minutes, for 45-55 minutes until deeply golden and jammy. Add wine to deglaze, scraping up any fond. Add thyme. Cook until liquid evaporates. Cool 10 minutes.
4. Whisk eggs with cream, half the Gruyère, and Parmesan. Season with white pepper and a grating of nutmeg.
5. Spread caramelized onions in the tart shell. Pour custard over top. Scatter remaining Gruyère over the surface.
6. Bake at 175°C (350°F) for 25-30 minutes until the custard is just set with a slight wobble at the center. Cool 15 minutes before slicing.

Cook's Notes: Do not rush the onion caramelization — those 50 minutes are what separates a great tart from a mediocre one. The tart shell can be blind-baked and onions caramelized a day ahead; assemble and bake fresh.