Preheat oven to 350 degrees F.

To prepare crust: Coat a 9- to 10-inch tart pan with cooking spray. Place all-purpose flour, whole-wheat flour, tarragon and 1/2 teaspoon salt in a food processor; pulse to combine. Add butter one piece at a time, pulsing once or twice after each addition, until incorporated. Add 1/4 cup oil and 1/4 cup yogurt and pulse just until the dough starts to come together. Transfer the dough to the prepared pan (it will be crumbly), spread evenly and press firmly into the bottom and all the way up the sides to form a crust.

Bake the crust until set but not browned, about 15 minutes. Let cool on a wire rack.

To prepare filling: Heat 2 tablespoons oil in a large skillet over medium heat. Add onion and cook, stirring occasionally, until tender, about 5 minutes. Stir in carrots and 1 tablespoon sherry (or rice vinegar) and cook, stirring, for 2 minutes. Remove from the heat.

Spread mustard over the crust. Sprinkle with Cheddar, then evenly spread the carrot mixture in the tart shell.

Whisk 1/2 cup yogurt, milk, eggs, tarragon, the remaining 1 tablespoon sherry (or rice vinegar), 1/4 teaspoon salt and pepper in a medium bowl. Place the tart pan on a baking sheet and pour in the filling.

Bake the tart until the filling is firm and the edges are golden brown, 40 to 45 minutes. Let cool for 15 minutes before slicing. Serve warm or chilled.

Ingredient note: Sherry is a type of fortified wine originally from southern Spain. Don't use the "cooking sherry" sold in many supermarkets--it can be surprisingly high in sodium. Instead, purchase dry sherry that's sold with other fortified wines in your wine or liquor store.

MAKE AHEAD TIP: Prepare the crust (Step 2), wrap tightly and refrigerate for up to 3 days. Loosely cover and refrigerate the baked tart for up to 1 day.

Equipment: 9- to 10-inch tart pan (with or without removable bottom)