In pressure cooking, onions soften and sweeten rapidly, often dissolving into the sauce, while sautéing allows for browning and caramelization, developing a deeper, more complex flavor.
When you sauté onions, you're cooking them over moderate heat in oil or butter. This process allows the natural sugars in the onions to caramelize, creating a rich, browned flavor. The onions soften gradually, retaining some of their texture. The browning, known as the Maillard reaction, contributes significantly to the depth of flavor.
Pressure cooking, on the other hand, uses high pressure and temperature to cook food quickly. Onions in a pressure cooker soften very rapidly due to the intense heat and steam. They tend to break down and release their moisture, often becoming almost translucent and dissolving into the surrounding liquid. While they still contribute sweetness and flavor, they don't develop the same browned, caramelized notes as sautéed onions. The high moisture environment favors steaming and softening over browning.
If you want to incorporate the flavor of caramelized onions into a pressure-cooked dish, sauté the onions separately beforehand and then add them to the pressure cooker. This will give you the best of both worlds: the depth of flavor from caramelization and the speed of pressure cooking.