Heat 1 tbsp olive oil and 1 tbsp butter in a large skillet over medium-high heat. Add the diced onion and cook for 3-4 minutes until softened. Add the sliced mushrooms and cook until browned. Stir in the minced garlic and dried thyme, cooking for 1 more minute. Add the spinach and cook until wilted. Remove from heat and stir in the shredded Gouda cheese. Season with salt and pepper.

Preheat oven to 400°F (200°C). Butterfly each chicken breast by slicing it horizontally, stopping about half an inch from the edge. Open it like a book and season the inside with salt and pepper.

Spoon the Gouda mushroom mixture onto one side of each chicken breast. Fold the other side over and secure with toothpicks if necessary.

Add a little more oil to the skillet over medium-high heat. Place the stuffed chicken in the hot skillet and sear for 3-4 minutes per side, until golden brown.

Transfer the seared chicken to a baking dish. Bake for 15-20 minutes, or until the chicken is cooked through and the internal temperature reaches 165°F (74°C).

Let the chicken rest for 5 minutes before slicing and serving. This helps keep it juicy.