For the Spice Blend: In a bowl, stir together all the ingredients.
For the Lentils: In a heavy 3 to 4 quart saucepan, melt the butter over medium heat. Add the onion, shallots, ginger, and a large pinch of salt, cover and cook, stirring occasionally, until the onion is soft, 8-10 minutes. Uncover, stir in the garlic and the spice blend, and cook, stirring, for 2 minutes.
Add the cabbage, potatoes, rhubarb, lentils, brown sugar, and bay leaf, along with enough cold water to cover by 1 inch. Raise the heat to high and bring to a boil, then reduce the heat and simmer gently, uncovered, until the lentils are tender, about 30 minutes. Stir occasionally and add more water as necessary to keep the dish fairly soupy.
When lentils are tender, season with salt to taste, stir in the peas, and simmer until the peas are just tender, about 4 minutes.