Position oven rack in the middle of the oven and preheat to 400°F Grease a 6-slot muffin pan with non-stick vegetable spray.
In a small saucepan, melt the butter. Add the onions and garlic and cook until soft.
In a small mixing bowl, whisk together the milk and eggs. Whisk in the butter mixture. In a separate bowl, whisk together the cornmeal, flour, baking powder, soda, salt, sugar and xanthan gum. Stir in the liquid mixture and stir to combine. Fold in bell pepper, jalapeño, corn and cilantro.
Divide the batter evenly among the muffins slots and bake for 20 minutes or until set.