Combine the first 3 ingredients in a medium bowl, and let stand for 5 minutes or until bubbly. Weigh or lightly spoon 6 ounces flour (about 1 1/4 cups) into dry measuring cups, and level with a knife. Stir 4 teaspoons olive oil into yeast mixture. Add flour, 1/4 teaspoon salt, and 1/4 teaspoon black pepper to yeast mixture, stirring until a soft dough forms. Knead dough on a lightly floured surface until smooth and elastic (dough will be soft and tacky).
Place dough in a large bowl coated with cooking spray, turning to coat top. Cover and let rise in a warm place (85°), free from drafts, 45 minutes or until doubled in size. Punch dough down; cover and let rest 5 minutes.
Drizzle 2 teaspoons oil into an 11 x 7inch glass or ceramic baking dish. Press dough into dish. Cover loosely with plastic wrap, and let rise in a warm place (85°), free from drafts, 30 minutes or until puffy; sprinkle dough with 1/4 teaspoon salt, nuts, onion, and thyme. Bake at 450° for 18 minutes or until golden.
Place arugula in a bowl; drizzle with remaining 1 tablespoon oil and juice. Toss. Turn bread out onto a work surface; top evenly with prosciutto. Slice bread into 4 (2 3/4 x 7inch) rectangles. Top each serving with about 1/2 cup arugula mixture; sprinkle evenly with remaining 1/8 teaspoon salt. Top with cheese.
I made this for guests as part of an Italian themed dinner - it was a huge hit!! I used 1 puff pastry sheet instead of making the dough. I added shredded fontina cheese, the walnuts, onion and thyme then baked for about 10 minutes. I let it sit until I was ready to serve. I also let the arugula sit on paper towels after I tossed it with the oil and juice so it would be too 'soggy'. So easy and partially make ahead.