Onion Tarte, Caramelized Onions, Bottarga

Ingredients

• 1/2 cup Lukewarm Water (105°F to 110° F)
• 1 1/2 teaspoons Dry Active Yeast
• 1 1/2 cups All Purpose Flour
• 1 1/4 teaspoons Salt
• 1 teaspoon Olive Oil
• 4 ounces (1 stick) Unsalted Butter
• 5 Yellow Onions (about 3 pounds), thinly sliced lengthwise
• Pinch of Salt
• 2 teaspoons Sugar
• 2/3 cup Crème Fraîche
• 1/4 cup Fromage Blanc or Plain Greek Yogurt
• 2 ounces (about) Bottarga (preferably from Sicily or Sardinia)
• Fleur de Del and freshly ground White Pepper
• 1 1/2 tablespoons fresh Thyme Leaves