Preheat oven to 400°F (200°C).
Prepare your ingredients: chop 10 oz. of the mushrooms and set aside; slice the remaining 4 oz. of mushrooms and set aside.
Slice pockets into the chicken breasts, being careful not to cut all the way through. Pound chicken lightly with a rolling pin to achieve uniform thickness. Set aside.
Heat 2 Tbsp. olive oil in a large skillet over medium to medium-high heat. Add the 10 oz. chopped mushrooms and sauté for 5 minutes.
Add the diced onion and sauté for 3 more minutes.
Stir in the parsley, garlic, thyme, salt, and pepper. Cook for another minute and then remove from heat.
In a small bowl, combine the mushroom mixture with 1 cup of shredded Gouda cheese. Stir well to mix.
In the same skillet, heat 2 Tbsp. olive oil over medium-high heat. Cook the chicken for 2 minutes per side, then remove from the skillet.
Stuff each chicken breast with the mushroom and cheese mixture and place them in a baking dish.
Bake for 50 minutes.
While the chicken is baking, heat 1 Tbsp. olive oil in the same skillet and sauté the sliced mushrooms for 2 minutes.
After 50 minutes, remove the chicken from the oven and carefully top it with the remaining cup of Gouda cheese and the sautéed mushrooms. Return the chicken to the oven.
Bake for an additional 7-10 minutes, or until the cheese is fully melted and bubbly.