Place the oil in a skillet set over medium heat. Add the onion and cook until very tender and light golden in colour, about 5 minutes. Stir in garlic and cook 1 minute more. Remove from the heat and cool to room temperature.
Place the sour cream and mayonnaise in a bowl and whisk to combine. Mix in the cooked onions and remaining ingredients. Transfer to a serving bowl. Cover and refrigerate until ready to serve. Can be made up to a day in advance of serving. If desired, you could garnish the top of the dip with a little grated gouda.
Recipe Options: Make cheddar onion dip, or asiago onion dip, by replacing the Gouda with either grated old cheddar cheese, or grated asiago cheese. Instead of parsley, give this dip and even more oniony flavour by mixing in 2 Tbsp. of very thinly sliced green onion.