Heat oil in a large nonstick skillet over medium-high heat. Add onion and cook, stirring often, until translucent and starting to brown, about 5 minutes. Pour in beer and increase heat to high. Bring to a vigorous boil; cook until the liquid is quite syrupy and the mixture reduces to about 3/4 cup, 8 to 10 minutes. Transfer to a large bowl. Stir in thyme, dry mustard, salt and pepper. Let cool for 10 minutes.