Boil the potatoes: Place the peeled and chopped potatoes in a large pot of salted water. Bring to a boil and cook until fork-tender (about 15 minutes). Drain well.
Mash the potatoes: Return the drained potatoes to the pot. Add 2 tbsp of butter, 100ml of milk, and salt to taste. Mash until completely smooth and creamy. Cover to keep warm and set aside.
Season the chicken: In a medium bowl, toss the chicken fillets with 1 tsp salt, 1/2 tsp black pepper, 1 tsp paprika, and 1 tsp dried Italian herbs until evenly coated.
Sear the chicken: Heat 2 tbsp of olive oil in a large skillet over medium-high heat. Add the seasoned chicken and cook for 5-7 minutes per side, or until deeply golden brown and cooked through (internal temperature of 165°F/74°C). Remove the chicken from the skillet and set aside on a plate.
Sauté the vegetables: In the same skillet, reduce the heat slightly and melt 2 tbsp of butter. Add the minced garlic and sliced onions, cooking for 1-2 minutes until fragrant and slightly softened. Add the sliced red, green, and yellow bell peppers along with the sliced mushrooms. Sauté for 5-7 minutes until the vegetables are tender-crisp.
Combine and serve: Return the cooked chicken to the skillet with the vegetables, letting it warm through for 1-2 minutes in the pan juices. Garnish generously with fresh chopped parsley.
Plate: Serve the hot garlic butter chicken and vegetables immediately over a generous scoop of the warm, creamy mashed potatoes.