If using sourdough starter, mix it with the warm water and let it sit for about 10 minutes. If using yeast, dissolve the packet in warm water with a pinch of sugar and let it sit for about 5 minutes, until foamy.
In a large bowl, mix the rye flour, whole wheat flour, salt, and seeds. Add the molasses and the sourdough starter or yeast mixture. Stir until well combined.
Transfer the dough to a floured surface and knead it for about 10 minutes, until smooth and elastic. The dough will be dense.
Place the dough in a lightly oiled bowl, cover it with a damp cloth, and let it rise in a warm place for about 1 hour, or until doubled in size.
Punch down the dough and shape it into a loaf. Place it in a greased loaf pan, cover it, and let it rise again for about 30 minutes.
Preheat your oven to 375°F (190°C). Bake the bread for 45-50 minutes, or until it sounds hollow when tapped on the bottom and has a deep brown crust.
Remove the bread from the oven and let it cool on a wire rack before slicing.
Enjoy your meal!