In a large bowl, mix to combine the Brussels sprouts, garlic, onion, eggs, and almond flour.
Season with salt and pepper. The mixture should be wet but not runny, and you should be able to form little palm-size balls out of it with your hands—if it’s too runny, add a little extra almond flour; if it’s too dry, add another egg or a splash of water.
In a large sauté pan over medium-high heat, melt the butter.
Working in batches so you don’t crowd the pan, carefully drop the fritter balls into the hot pan. Press them down with a spatula so they become more of a patty.
Cook for 3 minutes on each side, until tender and lightly browned, and serve.