Combine ham and onions; place about 1/3 cup down the center of each tortilla. Top with 2 tablespoons cheese. Roll up and place seam side down in a greased 13 x 9 x 2 in baking dish.
In a bowl, combine flour, cream, eggs, and salt, if desired, until smooth. Pour over tortillas. Cover and refrigerate for 8 hours or overnight.
Remove from the refrigerator 30 min. before baking. Cover and bake at 350 degrees F for 25 min. Uncover; bake for 10 min. Sprinkle with remaining cheese; bake 3 minutes longer or until the cheese is melted. Let stand for 10 minutes before serving.