Melt the butter and olive oil in a small saucepan. Add the onions and cook over medium heat until they begin to caramelize and take on a light brown color, about 8-10 minutes. Add a pinch of salt and the thyme, and stir into the onions. Remove from the heat and allow to cool.
Preheat the oven to 425° F. Combine the flour, baking powder, baking soda and salt in mixing bowl. Add the butter and blend with your fingertips or a pastry cutter until the butter is the size of small peas. Add the pecans and gently mix to incorporate. Combine the buttermilk and the caramelized onion, and add the mixture to the bowl. With a pastry scraper or a spatula, incorporate to form a shaggy ball. Do not overwork the dough. Turn out the ball onto a lightly floured surface. Dust the top of the dough with a little more flour and press or roll it to a ¾-inch thickness. Cut out the biscuits with a 1½-inch round cutter. Place the biscuits on a parchment-lined sheet pan. Press the scraps of dough together, and continue cutting. Bake the biscuits for 10 to 12 minutes until golden brown.
Meanwhile, in a small bowl, mash the blue cheese and the butter with the back of a soup spoon until it is of a spreadable consistency. When the biscuits are ready, cut them in half horizontally, and spread a little of the blue cheese in the middle.
Enjoy with a glass Cakebread Cellars Chardonnay Reserve.