For the dough, mix 50 grams (approximately 1/4 cup) flour and 50 ml (approximately ¼ cup) lukewarm water in a bowl. Cover bowl with plastic wrap and let stand and ferment for 24 hours at room temperature.
Add remaining wheat flour, rye flour and salt to bowl and mix well. Combine yeast, sugar and 350 ml lukewarm water and stir to dissolve and mix into dough. Knead with dough hook of electric mixer into a smooth dough. Cover dough and let rise in a warm place for about 45 minutes. Let go. Knead dough again. On a lightly floured work surface, divide dough into two pieces. Roll out each piece into an oval (about 30 x 50 cm)(approximately 12 x 20 inches). Fold long sides toward the center. Turn and fold dough lengthwise again.
Pinch seams of baguettes firmly together and place loaves with the seams down on 2 baking sheets lined with parchment paper. Cover and let rise in a warm place for about 45 minutes. With a sharp knife, make several shallow, diagonal cuts, about 0.5 c m (approximately 1/4 inch) deep on baguette surface. Brush baguettes with milk. Bake in batches in preheated oven at 220°C (approximately 425°F) for about 30 minutes, until golden brown. After 10 minutes of baking time, pour 1 cup of water on oven floor and shut oven door immediately. Cool finished baguettes on a wire rack and serve.