Dice the onion and all of the mushrooms. Thinly slice the garlic cloves.
In a cast iron skillet, heat 2 tbsp of olive oil.
Add the diced onions, mushrooms and half the garlic cloves inside the skillet, sprinkle 1 tsp of the salt and pepper and cook until wilted. Transfer to a large bowl and add the egg and almond flour. Mix well together. Take one of the rosemary sprigs and strip off the needles and mix them to the bowl.
Add your bird to the skillet, breast side up.
Lift the skin off the breast and stuff the breast with 3/4 of the mushroom stuffing. Make sure to stuff all the way (there is surprisingly lots of space beneath the skin of the breasts).
Brush the 3 tbsp of olive oil over the chicken. Add half of the garlic slices over the chicken and and rosemary. Sprinkle 1 tsp of the salt and coarse pepper over the chicken.
Bake in oven for 50 minutes.
Add the last 1/4 of the mushroom stuffing inside the skillet around the chicken. Bake another 20 minutes.
Take out of the oven and let cool for 10-15 minutes. Serve while it's hot!