In a skillet, heat the olive oil and butter over medium heat. Add the sliced onions and sauté for about 10 minutes, stirring occasionally, until caramelized and golden brown. Remove the onions from the skillet and set aside.
In the same skillet, add the mushrooms and cook for 5-7 minutes until they are browned and tender. Season with salt and pepper, then remove from the skillet and set aside with the onions.
Butter one side of each slice of bread. On the unbuttered side of two slices, layer the sautéed mushrooms, caramelized onions, and sliced Gouda. Top with the remaining bread slices, buttered side out.
Heat the skillet over medium heat and grill each sandwich for 3-4 minutes per side, or until the bread is golden and crispy and the cheese has melted.
Serve immediately and enjoy your gourmet grilled cheese!