Dissolve yeast and sugar in warm water in a large bowl; let stand 5 minutes. Lightly spoon flour into dry measuring cups, and level with a knife. Stir in 2 cups flour and 1/2 teaspoon salt until smooth. Turn dough out onto a floured surface. Knead until smooth and elastic (about 5 minutes), and add enough of remaining flour, 1 tablespoon at a time, to prevent dough from sticking to hands (dough will feel 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, 1 hour or until doubled in size. (Press two fingers into dough. If an indentation remains, the dough has risen enough.) Punch the dough down; cover and let rest for 5 minutes. Roll the dough into a 12-inch circle on a floured surface. Fit dough into a 10-inch round removable-bottom tart pan. Press dough against bottom and sides of pan.
Preheat oven to 425°.
Heat oil a large nonstick skillet over medium heat. Addthe onion; cover and cook 20 minutes or until golden brown, stirring frequently. Remove from heat; stir in 1/2 teaspoon salt, half-and-half, and 1 tablespoon thyme. Spread mixture over dough in pan. Bake at 425° for 25 minutes or until crust is golden brown. Sprinkle with 1 1/2 teaspoons thyme and pepper. Let stand 5 minutes. Cut into 6 wedges.