1. For the niter kibbeh (spiced butter): Melt the butter in a small saucepan over medium-low heat, swirling occasionally. Stir in the ginger, allspice, fenugreek, oregano, turmeric, cardamom, garlic and onions, and bring to a simmer. Simmer until the butter is clear and the milk solids remain on the bottom of the pan, about 30 minutes. Reduce the heat to low if the butter is boiling too quickly--if it burns it will taste bitter.
2. Line a strainer with dampened cheesecloth. Skim the foam from the top of the butter and discard. Ladle the butter through the strainer, leaving behind the milk solids on the bottom of the pan.
3. For the braise: Heat 3 tablespoons of the niter kibbeh in a large skillet over medium heat. Add the onions and a generous pinch of salt and cook, stirring frequently, until golden, about 10 minutes; reduce the heat as necessary as the onions cook down so they do not darken. Stir in the turmeric during the last minute.
4. Add the cabbage, potatoes, carrots and 1/2 cup water to the pan and stir. Season generously with salt and stir again. Bring to a simmer, cover, reduce the heat to medium low and cook, stirring occasionally, until the vegetables are very tender and almost falling apart, about 40 minutes. Check the mixture occasionally to make sure it doesn't dry out completely; if so, add a bit more water. The final dish should be buttery and moist, but not soupy. Season with salt and pepper and serve hot with injera.
This recipe makes more than enough niter kibbeh to make the braise. Refrigerate any leftovers to use in other Ethiopian dishes.