Season the Chicken: Season chicken breast pieces with 1/2 tsp salt and 1/2 tsp pepper. Let them rest at room temperature for 30 minutes.
Sauté the Onions and Cook the Chicken: Heat 2 tbsp butter in a large, deep skillet or pot over medium heat. Add the diced onions and cook until translucent. Add in the chicken and minced garlic, and stir to combine, until the chicken is cooked on all sides.
Add the Mushrooms and Combine: Stir in the sliced mushrooms and cook until they soften, mixing everything together.
Make the Sauce: Add tomato paste, sugar, and balsamic vinegar to the skillet. Stir to combine, letting the tomato paste cook slightly. Gradually pour in heavy cream or coconut milk, stirring to create a creamy sauce.
Simmer: Reduce the heat to low and let the mixture simmer for 10-15 minutes, stirring occasionally, until the chicken is cooked through and the sauce thickens. Adjust seasoning with additional salt and pepper to taste (I recommend 1 tsp salt and 1/2 tsp pepper).
Optional Crispy Potato Sticks: For a crispy topping, shred 1 russet potato, coat with cornstarch and a pinch of salt, and fry in hot oil until golden. Drain on paper towels. Alternatively, use store-bought potato sticks for convenience.