Season chicken cubes with salt and pepper. Melt 2 tablespoons of butter in a large skillet over medium-high heat, then sauté the chicken until fully cooked, about 10 minutes. Transfer the chicken to a bowl and set aside.
In the same skillet, melt another 2 tablespoons of butter over medium heat. Add sliced mushrooms and cook for 3-4 minutes until tender. Then add the remaining butter and minced garlic, cooking for about 30 seconds until fragrant.
Sprinkle flour over the garlic and mushrooms, stirring constantly for about a minute until the flour turns golden. Gradually whisk in the milk and chicken broth until smooth. Season with salt and pepper to taste.
Return the cooked chicken to the skillet and add the diced bell peppers. Mix thoroughly and allow to simmer for 2 minutes, then turn off the heat.
Preheat your oven to 350°F (175°C). Spread the mashed potatoes evenly over the chicken and sauce mixture. Level the top and create decorative lines with a fork if desired.
Bake for 45 minutes, or until the mashed potato topping is golden brown.
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Notes
Store leftovers in the refrigerator for 3-4 days. Reheat in the microwave or oven.