Grease one 9x5 inch (23x13cm) loaf pan. Set aside.
In a small bowl mix 1 tablespoon dry yeast and ¼ cup warm water and let foam for a few minutes.
In the bowl of the stand mixer, put 1 cup light rye flour, 1 cup white whole wheat flour, 2 teaspoons salt, ¾ cup buttermilk, 2 tablespoons malt syrup, 1 tablespoon honey, 2 tablespoons sunflower oiland yeast mixture. Mix with a wooden spoon until it integrates.
Attach the dough hook and start mixing on medium-low speed until a shaggy dough is formed.
Add 2 teaspoons caraway seeds, 1 teaspoon fennel seeds and 1 cup bread flour, a few tablespoons at a time.
Beat for 6 to 8 minutes, until the dough is elastic but not too stiff. If it's too sticky add a few more tablespoons of bread flour but don't overdo it. It needs to retain some stickiness.
Transfer dough to an oiled bowl, cover with plastic wrap, and let rise until doubled, about 1 hour. It can take more time depending on the temperature of your kitchen.
On a lightly floured surface, pat the dough down, and then roll it tightly, pinching edges to seal.
Place in the prepared pan, cover with a clean kitchen towel, and let rise again until doubled, about 1 to 1 ½ hours.
About 20 minutes before baking, preheat oven to 375ºF (190ºC).
Bake for 35 to 40 minutes, until golden brown and the bottom sounds hollow when tapped.
Let cool completely on a wire rack before cutting.