Make the spice mix: Put all the spices in a bowl and mix well. Set aside.

Make the stew: Put the red and yellow onions, garlic, and ginger in a food processor and process to a fine paste. (Do this in batches if your food processor bowl isn't large.)

Heat the oil in a Dutch oven or other large heavy-bottomed pot over medium heat. Add the onion paste and sauté, stirring occasionally, until the mixture has reduced a bit and turned light brown, about 20 minutes.

Reduce the heat to medium-low and add the tomato paste, 1 cup of the water, the 2 tablespoons berbere spice mix, and salt and cook, stirring and scraping the bottom of the pot occasionally, until the mixture thickens slightly, about 15 minutes.

Add the lentils and the remaining 4 cups water and stir well. Increase the heat to high and bring the stew to a boil, stirring occasionally, then cover the pot and reduce the heat to low, adjusting it as necessary so the stew simmers gently. Simmer, stirring occasionally, until the lentils are soft and cooked through, 35 to 40 minutes. Taste and adjust the seasoning with more salt if necessary. Serve hot.