Place water in large bowl; sprinkle with yeast and sugar. Let stand 3o seconds, stir until blended. Let stand 5 minutes or until yeast is foamy. Stir in whole wheat flour, vinegar and1 tsp salt until batter is smooth. Cover; let stand 15 minutes. Slowly stir in 1 cup of the bread flour to form soft dough. Cover; let stand 15 minutes.

Turn dough out onto lightly floured surface; knead 5 to 10 minutes or until smooth and springy, using additional 1/2 cup bread flour as needed to keep dough from sticking.

Lightly oil clean large bowl. Place dough in bowl; turn to coat with oil. Cover; let rise in warm place 1 hour or until doubled in size.

Meanwhile, melt butter in small skillet over medium heat. Add onion and garlic; cook 2 to 3 minutes or until soft. Remove from heat; stir in parsley and 1/2 tsp salt.

Lightly grease baking sheet or line with parchment paper. Turn dough out onto floured surface; divide in half. Pat or roll each half into 12 inch square. Spread each square with half of the onion mixture, layer with half the spinach. Sprinkle each with half of the cheese.

Tightly roll each dough square into log, enclosing filling. Pinch edges to seal. Place loaves, seam side down, on baking sheet. Cover with clean towel; let rise in warm place 45 minutes or until doubled in size.

Meanwhile, heat oven to 500°F. Spray loaves with water. With sharp knife, make 4 (1/4 inch deep) diagonal cuts in top of each loaf. Place bread in oven; immediately reduce oven temperature to 400°F. Bake 20 to 25 minutes or until lightly browned and bottoms of loaves sound hollow when slightly tapped. Remove from pan; cool completely on wire rack.