Special equipment: a spice grinder
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.
For the berbere (spice mix): While the butter is cooking, whisk together the chile powder, paprika, cayenne, ginger, salt, garlic powder, onion powder and nutmeg. Set aside.
Put the cardamom, coriander, fenugreek, allspice, cloves and cinnamon in a small skillet and toast over medium-low heat, shaking the pan regularly, until fragrant, about 4 minutes. Cool slightly.
Grind the toasted spices in a spice grinder to a fine powder. Add to the chile powder mixture and whisk to combine. Sift the spice mixture onto a piece of parchment paper, return to the bowl and whisk again. Return the pieces left in the sifter to the spice grinder and grind again as finely as possible; whisk into the spice mixture. Set aside.
To finish the niter kibbeh: 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.
For the lentils: Heat 2 tablespoons of the niter kibbeh in a medium saucepan over medium heat. Add the onions and cook, stirring, until golden, about 10 minutes.
Add 2 tablespoons of the berbere, the ginger and garlic to the pan and cook, stirring, for 1 minute. Add the lentils and 4 cups water, stir well and bring to a simmer. Reduce the heat to medium low and simmer, uncovered and stirring occasionally, until the lentils are very soft and the mixture is very thick (like a thick porridge or split pea soup), 35 to 40 minutes. (Monitor the heat to keep the lentils at a gentle simmer; the amount of heat needed may change as the lentils thicken.)
Remove the lentils from the heat and stir in the salt and 1/4 teaspoon of the berbere. Cover and let sit another 10 minutes. Serve hot with injera.
The lentils will thicken and mellow overnight, so feel free to make ahead and reheat before serving. This recipe makes more than enough berbere and niter kibbeh to make the lentils. Refrigerate any leftovers to use in other Ethiopian recipes.