Dissolve the yeast and sugar in the warm water. In a food processor or mixer, combine 1 cup of the bread flour, salt, 1/2 cup of the rye flour, the milk, egg, butter, Dijon mustard, onion, and caraway and sesame seeds. Add the yeast mixture. Beat in the rest of the rye and any of the remaining flour necessary to make a soft but firm dough. (It will still be slightly sticky.) Place in an oiled plastic bag or oiled bowl. Seal or cover and let rise until doubled (or refrigerate overnight). Punch down and shape into an oval. Place on a greased baking sheet. Let double. Preheat oven to 350 degrees. Brush the loaf with the egg glaze. Bake until the bottom sounds hollow when tapped, about 45 to 50 minutes. Remove from baking sheet and cool on a rack. The bread freezes, wrapped in plastic wrap and foil.