In a large bowl, mix together almond flour, coconut flour, xanthan gum, and salt.
Crack the egg into the flour mixture and add the cold water. Use your hands to knead into a smooth dough. If it's too dry, add a little more water. Wrap in cling film and let it rest while you prepare the filling.
Heat olive oil in a skillet over medium heat. Add cumin seeds and let them sizzle for a few seconds.
Add the onions and sauté until translucent. Stir in the ginger, garlic, and green chili, cooking until aromatic.
Add the ground chicken, breaking it apart with a spatula. Cook until no longer pink.
Mix in garam masala, turmeric, red chili powder, and salt. Cook for an additional 5 minutes, stirring occasionally.
Remove from heat and stir in chopped cilantro and lemon juice. Allow the mixture to cool.
Preheat the oven to 375°F (190°C) and line a baking sheet with parchment paper.
Divide the rested dough into 12 equal portions. Roll each portion into a ball and flatten into a circle on a piece of parchment paper.
Place a spoonful of the chicken mixture on one half of the circle, leaving a small border around the edge.
Fold the dough over the filling to create a half-moon shape. Press the edges together to seal, using a fork to crimp the edges.
Place the samosas on the prepared baking sheet.
Bake for 15 minutes, or until the samosas are golden brown.
Let them cool for a few minutes before serving.