Combine 1 cup flour, sugar, salt, and yeast in a large bowl. Add warm milk and margarine. Stir in starter. Mix in remaining 3 3/4 cups flour gradually; you may need to add more flour depending on the climate.

Turn dough out onto a floured surface and knead for 8 to 10 minutes. Place dough into a greased bowl, turn once to oil surface, and cover. Allow to rise until doubled in volume, about 1 hour.

Punch down dough; cover and let rest for 15 minutes. Divide dough in half and shape into 2 loaves. Place loaves on a greased baking pan. Allow to rise until doubled in volume, about 1 hour.

Preheat the oven to 375 degrees F (190 degrees C)

Stir together egg and water in a small bowl until well combined. Brush over the tops of loaves, then sprinkle with onion.

Bake in the preheated oven until loaves are golden brown and cooked through, about 30 minutes. An instant-read thermometer inserted into the center of a loaf should read 190 degrees F (88 degrees C).