Peel and finely dice the onion. Melt the butter in a skillet over medium, then gently sauté the onion. Add the crawfish, and sauté briefly.
Mix in the cream and the lobster stock, then bring to a boil. Reduce the heat, mix in the tomato paste, and stir to combine. Reduce the sauce by 2/3 over low heat. Season to taste with salt, pepper, and chili powder.