Preheat the oven to 180°C/360°F. Grease a 9x13 casserole dish with coconut oil or your preferred fat.
To peel a plantain, cut off its ends. Score the skin lengthwise, then grab a hold of the peel and pull it off without damaging the flesh (the riper a plantain is, the easier it will peel).
Grate the 6 plantains and the onion in a food processor, or on the largest holes in a grater. Once grated, strain the liquid in a large strainer, and pat with paper towels until fully dried.
OPTIONAL: If you would like to decorate your plantain kugel with whole plantains as pictured above, please slice one plantain into thin long strips and set them aside. Fry the slices in shallow oil until they are yellow and just starting to turn golden, about 3-4 minutes.
In a medium bowl, whisk all the eggs together. Add to the grated plantains and onions. Add garlic. Stir with a spatula until fully combined.
Add flour (or matzah meal), baking powder, salt, pepper, and coconut oil. Transfer to the greased casserole dish and spread evenly. If desired, top with halves of sliced plantains.
Bake plantain kugel in the oven, uncovered, for 45 minutes, until golden brown and a knife inserted in the middle comes out clean.
You can serve plantain kugel with traditional Colombian aji sauce, or with sour cream (dairy-free sour cream is fine too) with the zest and juice of half a lime grated into it.
Take care to wash your hands after dealing with plantains, as they can leave a sticky clingy layer behind.