Preheat the oven to 350 degrees Fahrenheit (175 degrees Celsius).
Gather the ingredients.
Line a large baking tray with a large baking sheet.
Peel and dice the onions into small cubes.
In a large nonstick frying pan, sauté the onions in olive oil until golden brown. Let cool and set aside.
In a large bowl, mix the rye flour, wholegrain spelt flour, oat flour, baking soda and salt until well blended.
Make a well in the center. Add the olive oil and vinegar in the middle.
While gradually adding the water, mix the ingredients together.
Knead the dough only until well blended, but don’t overwork the dough. It should be soft but not too sticky. If needed, add a slight amount of water or sifted rye flour.
Mix in the cooled sautéed onions.
Create round balls, about 2 inches - 5 centimeters in diameter. Tip: It helps to slightly moisten your hand with water before forming the balls.
Immediately place the balls onto a large baking tray lined with a large baking sheet, leaving room between them.
Gently flatten the surface a bit.
Cut a deep plus (+) in the center of each flattened ball with a knife or scissor.
Sprinkle with nigella seeds.
Bake for 18 minutes or until the dough is dry when tested.