Squash & Caramelized Onion Tart Gelson's

Ingredients

• 1 cup whole wheat flour
• 1 tsp kosher salt
• 1 cup all-purpose flour, plus more for surface
• 10 Tbsp unsalted butter, chilled and cubed
• ½ cup ice water
• 4 medium yellow onions, thinly sliced
• 3 cloves garlic, thinly sliced
• 2 Tbsp Dijon mustard
• Pinch cayenne pepper
• ¼ cup Gruyère, coarsely grated
• ½ small butternut squash, halved lengthwise, sliced ⅛-inch thick crosswise
• 1 delicata squash, halved lengthwise, sliced ⅛-inch thick crosswise
• 1 small sweet potato, halved lengthwise, sliced ⅛-inch thick crosswise
• 1 small red onion, halved lengthwise, sliced ⅛-inch thick crosswise
• Kosher salt, to taste
• 5 sprigs thyme
• 2 Tbsp unsalted butter