Use a mixture of at least 3 or 4 different varieties of onion. Suggestions include leeks, green onions, red or purple onions, white and yellow onions, cipollini, garlic, chives, and shallots. Prepare the onions and set aside.
Place a large, heavy bottomed skillet over medium-low heat and add the butter and oil. When the butter has melted, add the onions, stock, salt, pepper, and thyme.
Cover and cook, stirring occasionally, for up to 2 hours or until the onions have greatly reduced in volume and a deep golden brown color is achieved. Be sure to bring up any residue on the bottom of the pan each time you stir. An additional tablespoon of water or broth can be added at the end of cooking to deglaze the bottom of the pan. Remove the thyme stems.
Store covered in the refrigerator and use within 3 days. Or freeze and use within 3 months.