In a skillet over medium heat, melt the butter with the canola oil. Add the onions and saute with the garlic and leeks until translucent. Add the plantains and continue sauteing for 5 minutes. Add the bonito flakes and allspice and continue sauteing for 2 to 3 minutes. Transfer the mixture to a food processor and reduce into a coarse puree, keeping some texture to it. Add a little water if too thick. Season with salt and pepper.