Wash the spinach, shake it well in the sieve and pat dry with kitchen towels; it should not have too much water clinging to it. Chop roughly if the leaves are large.
Blend: Place it in the food processor with salt, egg, milk, soft butter, and sugar. Blend until smooth. Or use an immersion blender.
Combine: Mix both types of flour, yeast, and seeds.
Knead: If you can knead the bread in your food processor, add the mixture to the pureed spinach. Or knead everything in a stand mixer with the hands. Knead the dough well until smooth and not so sticky anymore. It took me about 7 minutes in the food processor.
Let rise: Lightly oil a large bowl, place dough inside, cover with a clean kitchen towel and let rise in a warm place until doubled in size, between 1 ½ to 2 hours depending on the temperature in the kitchen and the type of yeast you are using (Note 2).
Second knead: Punch down the dough. Transfer to a lightly floured working surface and knead 2-3 times with a small amount of flour to form a ball.
Shape: Press the dough into a thick rectangle almost as wide as your baking tin (my tin is 30 cm/ 12 inches, so I pressed the dough about 2 cm/ 0.8 inch smaller). Fold a third of the dough a third of the way over the rest of the dough. Fold the other side of the dough over the already folded piece. Pinch the seam.
Second rise: Butter the loaf tin and place the dough in it with the seam facing down. Cover with the kitchen towel again and let rise for about 30 minutes in a warm place.
Bake: In the meantime, preheat the oven to 180 degrees Celsius/ 360 degrees Fahrenheit. Bake for 55-60 minutes until golden on top and sounding hollow when tapped. Remove from the pan and let cool on a wire rack.