In a wide heavy-bottomed sauté pan, heat the oil over medium-high heat for 30 seconds. Add the onion and cook, stirring often, until soft and beginning to caramelize, about 7 minutes.
Add the potatoes and parsnips and stir until they are nicely coated with the onion. Add the stock, bay leaf, and salt.
Bring to a boil, cover, and reduce the heat to low