Walnut and Onion Tartine

INGREDIENTS

• 6 tablespoons warm water (100° to 110°)
• 3/4 teaspoon sugar
• 1 3/4 teaspoons dry yeast
• 3 tablespoons olive oil, divided
• 5 ounces all-purpose flour (about 1 cup plus 2 tablespoons)
• 5/8 teaspoon kosher salt, divided
• 1/4 teaspoon black pepper
• Cooking spray
• 1/2 cup coarsely chopped walnuts
• 1/3 cup very thinly vertically sliced red onion
• 1 ounce very thin slices prosciutto, chopped
• 1 teaspoon chopped fresh thyme
• 2 cups arugula
• 1 teaspoon fresh lemon juice
• 1 ounce Parmesan cheese, shaved (about 1/4 cup)